They made a big deal about that for a movie, considering it is all fantasy. It’s a distinction without practical meaning for storytelling purposes. Just to avoid the scene where Marty sees his own foto fade. They could have just ignored it.

A good way to think of this is a git checkout from anywhere . And then all future commits happen in a branch / detached head state, not affecting the place you checked out from i.e. original timeline.

They made a big deal out of it in the movie, that the stones had to be returned to the exact time they were taken from, or else they would be changing the future of the old "head".

That was exactly how it was used in Doctor Strange. It's just a movie, they play fast and loose with the rules all the time.