Bostrom is saying that if advanced civilizations have access to enormous amounts of computing power and for some reason want to simulate less-advanced civilizations, then we should expect that we're in one of the simulations rather than basement-level reality, because the simulations are more numerous. The simulator isn't an arbitrarily tacked-on detail; rather, it follows from other assumptions about future technologies and anthropic reasoning. These other assumptions might be denied: perhaps simulations are impossible, or maybe anthropic reasoning doesn't work that way---but they seem more plausible and less gerrymandered than traditional theism.