I think, Deathless is a word for abundant life. If we think as death then to no longer be constrained by his armies is to be freed to live fully. Gotama does not think of the deathless as immortality as the term is understood in Brahmanism but as the positive absence of reactivity.
The deathless isn't a thing, things are made up of bits and pieces of other things, and those things are made up of more things, things are endless and always in flux, this is life.. Buddha and the Brahmans discussed many things...hahaha
Buddha disagreed with the caste system, anyone could become enlightened, including women. Secondly he disagreed with animal sacrifice.
Here is a talk given to a Brahman
https://www.accesstoinsight.org/tipitaka/sn/sn51/sn51.015.than.html