FWIW, as far as I can remember I've always understood this the same way as Wei and cousin_it. (cousin_it was talking about the later logic-based work rather than Wei's original post, but that part of the idea is common between the two systems.) If the universe is a Game of Life automaton initialized with some simple configuration which, when run with unlimited resources and for a very long time, eventually by evolution and natural selection produces a structure that is logically equivalent to the agent's source code, that's sufficient for falling under the purview of the logic-based versions of UDT, and Wei's informal (underspecified) probabilistic version would not even require equivalence. There's nothing Cartesian about UDT.