It’s an assumption, with the LLM it is a fact. There is only one source of entropy, whatever the seed value is to any pseudo random number generator used for stochastic operations. Given those values, everything is deterministic. For the brain, it’s not at all obvious that this is true.
We have the LLM code.
You don't have the brain code.
I find it much stranger to believe that having/not-having the code would mean anything about consciousness.
I can believe it ... but only for a very extreme value of 'code'.