Turing machines are deterministic if all their inputs are deterministic, which they do not need to be, and if we allow them to be. Indeed, by default, LLMs are by default not deterministic because we intentionally inject randomness.

It doesn't mean we can accurately simulate the brain by swapping its source of nondeterminism with any other PRNG or TRNG. It might just so happen that to simulate ingenuity you have to simulate the universe first.