But UDT’s decision on how to interact woth Omega does direct affect worlds in which Nomega exists instead of Omega.
Again overly simplistic prior:
50% chance: Omega exists, and we get counterfactually mugged, half of the times heads and half of the times tails.
50% chance: Nomega exists, guesses what we would do if Omega existed and the coin came up tails, and pays out accordingly.
There is only one decision—do you pay if Omega exists and the coin comes up tails, and that decision affects both (or all three) possible worlds.
Even once you see that Omega exists, UDT already recognized that in order to maximize utility it should precommit (or just decide or whatever) to not pay.
I don’t think Nomega has to simulate you interacting with Omega in order to know how to would react should you encounter it, in the same way that you can predict the output of many computer programs without simulating them.
By the time you get mugged, you could be 100% sure that you are in the Omega world, rather than the Nomega world, but the principle is that your decision in the Omega world affects the Nomega world, and so before knowing UDT commits to making the decision that maximizing EV across both worlds.
This logic operates in the same way for the coin coming up tails—when you see the tails, you know your in the tails world, but your decision in the tails world affects the heads world, so you have to consider it. Likewise, your decision in the Omega world affects the Nomega world (independent of any sort of simulation argument).
This argument would also suggest that by the time you see tails, you know you live in the tails world and thus should not pay up.