My thinking here is that if a being suddenly shows up and can perfectly model me, despite not having scanned my neural pathways, taken any tissue samples, observed my life history, or gathered any other data whatsoever, then it's cheating somehow -- i.e. I'm a simulation and it has my source code.

This doesn't require there to be a more real Prismattic one turtle down, as it were. I could be a simulation created to test a set of parameters, not necessarily a model of another entity.