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.
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.