The set of all simulations is irrelevant in this case. What matters for us is the set of simulations that match our observations. For this set, historical simulations of various forms are naturally expected to be predominate.

The past can't simulate the future, so we must be in a sim from a future timeline. Loosely speaking this leaves open historical sims and 'fictional' sims. From the inside they may be hard to differentiate (consider that harry potter's world looks historical from his perspective, etc.)

If multiple levels of sim are likely, I have a simple argument that fictional sims are more likely than you'd think: for us to be in a historical sim with respect to the root physical universe, every sim level in the stack must be historical. If even one sim in the tree/stack/chain is fictional, then everything below that level is also fictional.

So 'fiction' is something that only increases with sim levels.