I guess logically one needs to assume something like if you simulate the brain completely accurately the simulation is conscious too. Which I assume bc if false the concept seems outside of science anyway.

Let's imagine a world where we could perfectly simulate a rock floating through space, it doesn't then follow that this rock would then generate a gravitational field. Of course, you might reply "it would generate a simulated gravitational field in the simulation", if that were true, we would be able to locate the bits of information that represent gravity in the simulation. Thus, if a simulated brain experiences simulated consciousness, we would have clear evidence of it in the simulation - evidence that is completely absent in LLMs