They took the Sliders theory. Now, because Steve is Steve, he pretty much has to dimension hop to Avengers Homebase, bring Sam the Shield and give him the Legacy because he's living in another continuity, the result of all the Buckys, Howards, Peggys and occasional Steves he's interfered for. Well, his continuity is only based on one of those though he gets there because of making a change and then time traveling to a related bit of multiverse.

It's a much more constrained sort of travel than the Parallel Reality conceit, where you get Very Different Tonys for example.