I don't personally suspect that anyone could truly create a simulation of their own universe at all. You could absolutely simulate a universe, but simulating your own universe (presumably your own universe at a point in the past since that's what context the simulation argument generally gets made in) would have to have some kind of deviation from the real universe, be it that not all of the universe is simulated, or it's only simulated to a certain level of detail or "resolution" and any physics on a smaller scale is simplified, or time runs slower or something. Because if you can simulate a perfect copy of your universe, or a universe of equivalent complexity and speed, then you can build a computer in that simulation equivalent to the one running it, and since that simulated computer doesn't use all the resources of it's simulated universe presumably, you can build several of them and get more processing power than you started with, which makes no sense. And if every "layer" of simulation inheritly has dramatically less possible complexity to it than the layer above, you should eventually (and I suspect rather rapidly) reach a level where further nested simulations are not possible