I don't think your argument follows. Assume there's an uncreated universe that is very suited to running simulations. Assume someone in that universe creates our universe, fully contained inside theirs. Yes, that person is outside
our time, and started
our time, but in their universe, they are 1) constrained by physical space 2) not all-encompassing at all 3) have a whole lot of stuff in there with them that is external to them.
Sure, you can go into infinite regress arguments, but they hit both of our arguments just as hard.