I really love these Markus and McFeely interviews that are making the rounds, and how hard they are sticking to heir guns saying that Cap indeed lived his life in the MCU timeline. They even go out of their way to say that was the intention since The Winter Soldier. Since the film doesn't really contradicts that notion, only the Russos, I'lol sticking with them, since it makes sense:
I think the explanation that Markus and McFeely are sticking to Cap living in the same timeline is that their job is to create the time travel logic in their story to be as self-contained as possible, so The Ancient One's line about creating a branch reality by removing a stone is their way of putting a set of rules for what can create a branch for this movie as well as what will allow them to "clip the branches" without the heroes' actions creating all kinds of problems by traveling through time. The issue is that they also make it very clear that the act of traveling to your past essentially creates a branched reality because any thing you do in your past can't effect your future, and that Back to the Future rules don't apply.
Steve staying in the same timeline means that Back to the Future rules may apply, since he'd essentially be rewriting the past by existing in it. It's interesting that Markus and McFeely were thinking about Steve having always been Peggy's husband and traveling through time being always something he was meant to do, and just that they'd kept that fact secret for years, but that also creates implications as much as him creating an alternate branch, as the Russo's have stated.
It's interesting that the movie lays out evidence for both, and seemingly contradicts itself, even in ways that Markus and McFeely specifically seem to have written in themselves, so it's not like the Russos and Kevin Feige high jacked the script to contradict them and make alternate realities still possible. I just think M&M's goal wasn't to set up alternate realities, just to find a way to introduce time travel and place the lid back on that can of worms as best as they could, but the can of worms is opened. In that sense, I think the Russo's interpretation makes more sense to me going forward than what M&M believe in the context of the film since their rules suggest that branching paths will occur regardless of removal of a Infinity Stone from a timeline, so The Ancient One may be either mistaken or was talking less about it being the only way to create a branch and more about how removing the Time Stone specifically from her perspective could have disastrous consequences due to how powerful it is, which sounds fair, because just the implications of collecting Infinity Stones from all different timelines to put in one timeline would really screw with reality.
We already see Loki getting the Space Stone and that seems like it would have huge implications for that timeline and Strange already visiting millions of potential futures that seemingly would also be timelines that would have to exist in order to visit them, since if they had one fixed timeline, you can't visit potential futures if the present hasn't been written yet. Especially since the way that Strange talks about the futures would make it seem like they are determined in their outcome with Tony's survival and then snap being the one outcome that is different among millions of iterations of the future.
They opened that can of worms, and it would seem that while M&M wouldn't have intended for there to be loose ends, Kevin Feige likely was counting on it which the implications we've seen in Far From Home (if Mysterio is telling the truth) as well as what it may mean for Loki's series, presuming we follow him on his Space Stone shenanigans, and also if they ever decide to revisit Steve's journey in replacing the stones, they'll have a chance to definitively choose and explanation, that may not fit with what Markus and McFeely intended.
In the Yahoo video, Markus and McFeely even point out that Steve would travel to 1948 after the events of Agent Carter, but that isn't something Steve would know about and shoot to catch Peggy on the rebound after her and Sousa didn't work out. That's just M&M trying to avoid any canon conflicts, which they've tried to weave around to the best of their ability. It's simply on the writers/creatives that pick up the baton from them to decide what actually happened.
