Well, I think it's less time travel and more an alternative universe, which of course is what the new Dr Strange film will be dealing with.
Yes, I think that's it exactly.
Endgame introduced the idea of "alternate timelines." The rooftop conversation between Bruce and the Ancient One sort of suggested that while timelines could coexist for a little while, they were also fragile . She was worried her reality would cease to exist if even one Infinity Stone was removed*. That's why Steve had to return the Infinity Stones to their original places**.
iirc, the idea of a Marvel 'multiverse' was technically introduced in
Spider-Man: Into the Spider-Verse, but at the time it was presumed that movie was just a one-off and not part of MCU canon (it was a Sony movie, and the fact that it was an animated film probably helped to think of it as separate from the other Spider-Man movies***). I think the title of
Doctor Strange in the Multiverse of Madness is what introduced the term 'multiverse' to the MCU.
Ant-Man and the Wasp: Quantumania reinforces the idea that the MCU is playing with a "pop sci view of the many-worlds interpretation of quantum mechanics." They're definitely combining 'alternate timeline' and 'alternate universe/dimension' into a single concept that isn't well-defined yet.
Okay, here's another brain-twister
(
Endgame and
Loki spoilers):
* At the time, the question was whether the Time Stone was unique in that way. While the Ancient One clearly had supernatural senses, it was unclear whether she knew that Bruce's colleagues were busy stealing other Infinity Stones. It also likely that the Ancient One was aware of alternate timelines vanishing shortly after the event that caused them to branch off, but didn't know why. She may have assumed that was the natural end of branching timelines, but we know now that it wasn't natural at all, it was the TVA "pruning" them.
** iirc, Steve also took the Mjolnir that Thor had stolen from himself. We know now that the sudden disappearance of Mjolnir around the time of
Thor 2 would probably have created a "Nexus Event."
*** Although that conceptual division between the live-action and animated stuff may be dissolving:
What If..? might be a series of vignettes showing us alternate, canonical timelines, featuring those whose stories won't ever tie back into the main MCU.
