The point is that a physical simulation of the universe includes a description of A. If A simulates the universe, it sees that its outputs appear on W2 without postulating anything beyond physical law. (This is all most straightforward in the case where A does perfect induction and the shortest explanation for A's observations takes the form of pointing to A in the universe, which is the one I started with; if A is capable of executing human-equivalent reasoning, though, the same thing happens in less easily formally analyzed universes.) I will try to edit the post to be more clear.