the program corresponding to 'universe A simulating universe B and I am in universe B' is strictly more complex than 'I am in universe B' while also predicting all the same observations, and so the 'universe A simulating universe B' part of the program makes no difference in the same way that mental epiphenomena make no difference - they predict you will make the same observations, while being strictly more complex.

True, but, just as a reminder, that's not the position we're in. There are other (plausibly necessary) parts of our world model that could give us the implication "universe A simulates us" "for free", just as we get "the electron that goes beyond our cosmological horizon keeps existing" is an implication we get "for free" as a result of minimal models of physics.

In this case (per the standard Simulation Argument), the need to resolve the question of "what happens in civilizations that can construct virtual worlds indistinguishable from non-virtual worlds" can force us to posit parts of a (minimal) model that then imply the existence of universe A.