SN 48.9 what is "release as object" referring to for samadhi-indriya?

https://buddhism.stackexchange.com/questions/33109/how-to-let-go-in-meditation/33237#33237

How to 'Let Go' in Meditation?
"And what is the faculty of concentration? There is the case where a monk, a disciple of the noble ones, making it his object to let go, attains concentration, attains singleness of mind.
Indriya-vibhanga Sutta (SN 48.10)
What is exactly meant by letting go & how is this exactly done (in daily life & meditation) ?
Why is the breath not the 'object', if it's called Mindfulness of Breathing?

Nothing wrong with B.Thanissaro's translation you quoted, but here is Bodhi's for comparison, which makes something more clear:

SN 48.9

“And what, bhikkhus, is the faculty of concentration? Here, bhikkhus, the noble disciple gains concentration, gains one-pointedness of mind, having made release (vossagga-arammanam) the object.194

This is called the faculty of concentration.

vossagga, here is primarily referring to the release, "letting go", that is nirvana. That is, we use the samadhi faculty for the purpose of attaining nirvana. The pali grammar is ambiguous here, so it could have a secondary meaning in addition to the above, that the process of release or letting go is what we depend on to attain successively deeper levels of samadhi. But the primary meaning of that sutta passage is that the purpose of samadhi is to realize nirvana. That's what Bodhi's footnote explains the commentaries and exegetical tradition says, and in this case I agree with it, because of where vossagga appears in the 7 awakening factors formula, after "viraga nissitam, nirodha nissitam, vossagga-parinamim). Whenever you have a sequence with viraga and nirodha, the next term, whether it's vimutti, or some other word, is going to refer to the realization of nirvana.