It’s not that the act of travelling back in time creates a new universe, the time travellers simply go to a parallel universe that’s similar to the one they came from, up to the point where the time travellers appear.
I disagree on this point. I think the act of traveling through time is what creates the alternate universe, which diverges at the point The travelers arrive.
Though I’d argue that, based on the obvious pre-Kelvin differences in ST09, Spock was slightly mistaken and he and Nero traveled to an already-existing alternate universe in addition to traveling back in time. But that’s mostly head-canon.