we discover that many of the counter-arguments that we advance against theist apologetics are (objectively speaking) equally effective against simulationist speculation.

From what I've seen, the primary argument for simulationism is anthropic: if simulating a whole universe is possible, then some entity would do it a lot, so there are probably a lot more simulations out there than "basement realities", so we're probably in a simulation. What effect MWI has on this, and what other arguments are out there, I don't know.

Typical atheist arguments focus on it not being necessary for god to exist to explain what we see, and this coupled with a low prior makes theism unjustified--basically the "argument from no good evidence in favor". This is fine, because the burden of proof is on the theists. But if you find the anthropic argument for the simulation hypothesis good, then that's one more good argument than theism has.