I think that using the uniform prior over observers constitutes a critical learning failure. Calling such beliefs "true" or "false" seems to be presupposing too much philosophically.
Note that AIXI doesn't do this; it competes with every predictor, including predictors that reject the simulation argument for one reason or another (some of which are quite simple). We can debate whether it gives 50% or 99.999% or whatever probability to being in a simulation. But we can hopefully agree that it gives less than 99.99999999999999999999999999% probability.
Existing techniques probably won't super-confidently accept the simulation argument either.