Explanation from B. Thanissaro regarding Ajahn Mun biography section where he has conversations with (living) Buddhas

Ajahn Mun Bio, account of conversation with Buddhas

excerpt from p.172 Venerable Ãcariya Mun Bhýridatta Thera: A Spiritual Biography by Ãcariya Mahã Boowa Ñãõasampanno.  http://www.forestdhamma.org/ebooks/english/pdf/Acariya_Mun__A_Spiritual_Biography.pdf

On the nights subsequent to Ãcariya Mun’s attainment of vimutti, a
number of Buddhas, accompanied by their Arahant disciples, came to
congratulate him on his vimuttidhamma. One night, a certain Buddha,
accompanied by tens of thousands of Arahant disciples, came to visit;
the next night, he was visited by another Buddha who was accompanied
by hundreds of thousands of Arahant disciples. Each night a different
Buddha came to express his appreciation, accompanied by a different
number of Arahant disciples. Ãcariya Mun stated that the number of
accompanying Arahant disciples varied according to each Buddha’s
relative accumulation of merit – a factor that differed from one Buddha
to the next.

Many critics cite that passage to show that Ajahn Mun could not be an enlightened Theravadin monk.

For how could an Arahant converse with still living Buddhas who had pari-nirvana'd already? 

Ven. Thanissaro, of Wat Metta  comments on the Buddha Conversation incident

I asked Ven. Thanissaro, of Wat Metta, ordained in Ajahn Mun Dhammayut Thai Forest tradition, he explained:

As for the Ajaan Mun visions, you have to remember his general attitude toward beings encountered in visions:  the important part of the vision is not who delivers the message, but what the message is —

 

whether it’s in line with the Dhamma, and what results it gives when you put it into practice.

 

We don’t know whether Ajaan Mun actually thought he was conversing with arahants. All we have are third-hand reports. In the case of the on-line discussions, they are based on a further remove, translated into English, a language that is less ambiguous than Thai.

 

When one says in Thai that one saw so-and-so in a vision, it doesn’t mean that one necessarily believes that so-and-so actually came for a visit, simply that that was how the vision appeared.

 

There are many ways of explaining how such a vision could occur — arahants in the Pure Abodes are clearly one possibility, devas who had known arahants in the past are another. I don’t find on-line discussions particularly fruitful.

Hāsapañño Bhikkhu comments on the Buddha Conversation incident

Hāsapañño Bhikkhu, owner of website : https://sites.google.com/site/wideanglewilderness about the same , here's what he says about it :

"
I’m sure you are aware that they fall almost completely within ‘Buddha-visaya’ and 'Jhāna-visaya’ (things that fall within the range of experience and possibilities for Awakened Ones, and within the range of experience and possibilities for people in jhāna) — i.e. 2 of the 4 things that cannot be encompassed or understood at the level of thought (from the Acinteyya Sutta [A.4.77]). It seems to be universally held among the Thai kammatthana tradition (and also among other Buddhist traditions) that arahants and Buddhas of the past will come and help people who are truly practising for real.

This is usually just referred to ‘matter-of-factly’, without any attempt at interpretation or explanation - like in Ajahn Mun’s biography. The only detailed explanation of these phenomena that I have heard also comes from a great Ajahn who was once asked that if these things happen, whether arahants or Buddhas somehow still ‘exist’. He replied that these phenomena of post-parinibbāna arahants or Buddhas was something like relics… In the same way that physical relics of the Buddha, etc., can still exist, enlightened beings can use their samādhi to leave behind mental relics (relics of nāma-dhammas rather than rūpa-dhammas) that meditators of succeeding generations can tune into or come into contact with, even after the arahants have passed into parinibbāna. And since they are nāma-dhammas, they are interactive in a strange way that physical relics obviously are not. So it is not the case at all that the Buddhas or the arahants are 'coming out of Nibbāna’ to visit people, even though it seems like the knee-jerk assumption that a lot of people jump to…

Ajahn Mun no doubt had these experiences in his meditation — as have many, many other monks and nuns and laypeople as well — and it is best to keep an open mind about it without trying to assume you understand what is going on, and without jumping to conclusions or interpreting or explaining what you don’t understand.

None of this contradicts the Brahmajala Sutta. The Buddha did state, however, that “He who sees Dhamma, Vakkali, sees me; he who sees me sees Dhamma. Truly seeing Dhamma, one sees me; seeing me one sees Dhamma.” [S.22.87] There is certainly more than one way to interpret that, and more than one interpretation could be correct...
Anyway, I wouldn’t let it stand in the way of following the teachings of these Ajahns. Taking an intellectual position against them because of it would be a strange hill to die on for someone interested in Dhamma.

Very best wishes,
Hāsapañño Bhikkhu "

Ajahn Chob was meditating nearby when Ajahn Mun attained arahantship

Found this text here describing Ajahn Mun's meeting with the devas after Arahantship.

audio recording of same text here:

https://archive.org/details/ajahn-chob-witnesses-ajahn-mun-become-arahant

Ācariya Mun Bhūridatto attains to the Dhamma-element

Luang Pu Chob

– translated by Thiracitto Bhikkhu

When Venerable Luang Por Chob Ṭhānasamo and Venerable Luang Por Mun Bhūridatto were staying in the Chiang Mai area practising meditation together at the Dok-Kham cave near Sahagorn Village, Nam-Phrae Township, Phrao District, an event of utmost significance occured. Although Luang Por Chob was not directly involved in the event, he bore witness to the crucial moment when Luang Por Mun finally realized the Dhamma-element, that is, the moment when Luang Por Mun realized the path and fruit of the Buddha's teachings and ultimately became one of the Arahants of our time.

It all happened on a day in May 1935, around 3 a.m. As Luang Por Chob was sitting in deep meditation, his mind still and sublime, shining brightly immersed in samādhi, all of a sudden, the brightness and stillness of his heart was blown away in an instant. That very moment, he heard a tremendously loud resounding thunderclap, so massive that everything started to tremble and shake. Later on, Luang Por Chob would describe this bang as being as if a nuclear explosion had gone off right next to him. The force was so powerful that everything started to shake as if the whole world was breaking apart and dissolving into the tiniest particles. The whole scene, however, was unfolding solely within his own mind; the outside world remained the same.

Never since he had begun practising, had Luang Por Chob experienced a meditational phenomenon like this. Inwardly examining his mind, he tried to figure out what was responsible for this mindstate. But in the end, he couldn't discover anything unusual. Therefore, he directed his investigation outwards to inquire as to the reason for such a phenomenon to have occured. As soon as he sent his mind outward, it shot straight towards the nearby dwelling place of Luang Por Mun. There he witnessed (with his mind’s eye) multifarious male and female devas crowding around Luang Por Mun. There were more heavenly beings than he had ever seen before. Hordes of devas from every single heavenly plane gathered to such an extent that the whole of space - from heaven to earth -  seemed to be completely saturated. Devas from more than a billion galaxies clustered around Luang Por Mun. The brilliance of their luminosity shone so brightly that the entire surroundings of the Dok-Kham cave were flooded with light, fully illuminated by the powerful radiance of virtue. And yet, the luminosity of all the deva gods combined was nothing compared to the mesmerizing brilliance and majestic beauty of Luang Por Mun's own heart; the blazing radiance of his Dhamma perfection exceeded all else. Luang Por Chob would later report that Luang Por Mun's grandeur and beauty was so much brighter and more majestic than could possibly be described in words.

After admiring Luang Por Mun's dazzling brilliance for a sufficient period of time, Luang Por Chob pulled back his mind and thought to himself: "This is certainly due to an important occurrence in the Great Ajahns practice today. I'm quite sure about it!”

The very next day, around 4 p.m., Luang Por Chob went to Luang Por Mun's dwelling place to prepare the wash water for his daily bath. Just as Luang Por Chob was handing his teacher a fresh bathing cloth, Luang Por Mun asked him: “Tan Chob, how is your practise lately?” Luang Por Chob replied respectfully: “My practise is as usual, Venerable Ajahn. But yesterday something happened in my meditation that almost literally blew my mind away. There was an explosion, a bang as loud as if a huge bomb had detonated right next to me. I've never experienced anything like it in my meditation.” Luang Por Mun then asked: “How do you assess the matter?” Luang Por Chob replied: “I started looking for causes in my heart, but couldn't find anything unusual. But when I directed my mind outwards, immediately I saw an incredibly wonderful light emanating from you, more radiant than I have ever seen before.  And on top of that, I have never seen such multitudes of devas gather around you, Luang Por. But still, I don't know why so many devas would have visited you on such a scale.”

Luang Pu MunLuang Por Mun answered: “The multitude of devas you saw yesterday gathered in such enormous numbers to express their Anumodanā - their emphatic joy and appreciation - to me. They congratulated me for realizing the Dhamma-element - the complete liberation from suffering and the end of the samsaric transmigration. From now on, there is no further birth for me. Everything is cut off completely. To the extent that an Arahant is without delusion, that's exactly how I am now without delusion. The explosion you heard was caused by the devas' saluting proclamation of appreciation and recognition. The Anumodanā-blast created by their psychic power, in all likelihood, impacted your mind so violently, that it was properly shaken and catapulted you out of samādhi."

Luang Por Chob later tells us: “At that moment, all of my hair stood on end. I would never have dreamt to ever hear such a statement from Luang Por Mun personally, nor – up to that day – had I ever heard of this term ‘Dhamma-element’ in my entire life. But it kindled such a mental delight (pīti) that all mindfulness and self-control escaped me and tears began streaming down my face. In that moment, I could not find words appropriate enough to do justice to that highest level of Dhammic realization which my revered and beloved Master - who was like mother and father to me - had attained. So I fell on my knees in front of him and bowed low to his feet. Kneeling with my chest bent down, staring at his feet, I started crying profusely without any inhibition. I was so stunned and overwhelmed by the fact that he had penetrated to the Dhamma-element and had become a fully-awakened Arahant. Never had I cried so much in my life. I don't know how to describe the feelings in my heart that day. It is too deep a spiritual experience and defies any description.

“Up until that day, Luang Por Mun was generally very strict and would reprimand me if I lost my composure. But however harsh he scolded me, I always took it to heart (that's why I had expected a harsh admonishment on that day). But now he said nothing. He just let me cry, let me express my feelings freely. When I calmed down after a while, Luang Por Mun said to me: "Well, look at that one, Tan Chob!  See how far the mind can go!  Things can be so profound that your mind is so overwhelmed by spiritual joy that it even makes you sob! Isn't that right Tan Chob?" I replied to him: “I am speechless at the idea that you, Luang Por, have finished your work in the Buddha's teachings. That you have overcome all suffering and that now I am left behind and still have to keep practicing. I wish to understand the Dhamma - to see the Dhamma - in this very life, just like you Luang Por!"

Luang Por Mun responded, “If you want to achieve path, fruit and Nibbāna in this life, under no circumstances slacken your efforts. It takes unremitting dedication for the practise. No slacking up, no matter what. Practise as I have taught you so far. If you practise exactly as I have instructed, you will realize to your complete satisfaction path, fruit and Nibbāna in this very life. That, I guarantee you.”