Except they most certainly don’t treat it as a single timeline. If they had, they would have just gone back and stopped Thanos. Instead we got Endgame with multiple timelines that makes bringing stuff to the future consistent with how they say it works. Things happen in a different way in the branched timelines and nothing in the time traveler‘s past is changed.
Why would there need to be a mention of that? It kind of follows from the way they say time travel works. The other timelines we get to see in Endgame are in the past of the main timeline we followed, they don’t branch off until the Avengers go back to them, of course everything happens exactly the same way until that point since that’s what they went back to.
Let’s say the Banner-Stark Time Machine works like this:
- You can go to any location and any point in time in any timeline.
It may not be explicitly mentioned, but it is as far as I know also not contradicted by anything explicitly mentioned so far. Awesome, suddenly almost everything in Endgame works flawlessly!
Plausible constraints could be that one needs to be able to navigate this infinite set of spacetime coordinates. Maybe the primitive navigation system of the Banner-Stark Time Machine only allows travel to:
- Any point (timeline + location + time) previously navigated to/from.
- Any point (location + time) in the past of the same timeline. (Immediately branches off to a new timeline upon arrival)
That would be a reason not to mention the machine’s ability to cross between timelines as they lack the more sophisticated navigation tech the TVA got, while allowing the Avengers to visit the past, return to the main timeline they started out from and to return the borrowed infinity stones to the correct timelines.
So instead of basing the MCU time travel on what they say and do in the MCU, you are trying to force the MCU time travel to fit other theories?
No, it is not magic jumping in Loki, just technology. If you accept that the TVA can do it, you have to accept that others in the MCU could potentially do it as well.