Because that's how the rules of the system we exist within operate more generally.

We've done similar experiments with more controlled/simple systems and physical processes that satisfy the same symmetries needed to make that statement with rather high confidence about other similar but much more composite systems (in this case, humans).

It's more like saying, in principle, if a bridge existed between Mexico and Europe, cars could drive across. I'm not making any new statements about cars. We know that's true, it would just be an immense amount of effort and resources to actually construct the bridge. In a similar vein, one could, in principle, build a device that somehow stores enough information at some precision needed to arbitrarily predict a human system deterministically and do playback or whatever. Just, some levels of precision are harder to achieve than others in terms of building measurement device complexity and energies needed to probe. At worst, you could sample down to the uncertainty limits and, in theory, reconstruct a similar set of behaviors by sampling over the immense state space and minimizing the action potential within the simulated environment (and that could be done efficiently on a large enough quantum computer, again, in principle).

However, it doesn't seem to empirically be required to actually model the high levels of human behavior. Plus, mathematically, we can just condition the theories on their axiomatic statements (I.e., for markov blankets, they are valid approximations of reality given that the system described has an external and internal state, a coherence metric, etc etc), and say "hey, even if humans and LLMs aren't identical, under these conditions they do share, they will have these XYZ sets of identical limit behaviors and etc given similar conditions and environments."