That's an unsubstantiated belief, though.

If we simulate a rock right down to the quantum level - not even remotely possible with foreseeable technology, but let's assume it is - you don't end up with an actual rock in the physical universe. You end up with a model of a rock. There are important properties that it's missing due to not having a direct physical embodiment beyond its model. You can't bash someone in the head with it in the physical world, for example.

In the consciousness case, you're assuming you can "get around this" by using communication between the model and the host universe - communication could allow a model a way to have an effect beyond its host environment, unlike the simulated rock.

But this involves a lot of assumptions about the nature of consciousness, its physical basis, how a model of consciousness would behave, etc. Given the number of unknowns here, the claim that "it's theoretically possible" is supported just as well as the claim that it's theoretically impossible - which is to say, not at all.