What Hulk explains is that you can't travel back in time in the same timeline. But what you can do is travel to alternate timelines via the quantum realm. This is actually the prevailing theory of quantum mechanics which says that at the quantum level, events are not fixed but rather exist in a state of infinite possibility and each possibility is represented by an alternate universe.
So if they go back and kill Thanos, what they would be doing is killing a Thanos from an alternate reality and the reality in which Thanos wins would still exist. So the plan was to go back in time and recover the stones from an alternate reality then bring them back to the present and use the Infinity Gauntlet not to undue the past which they can't but to bring everyone that died from the snap into 2023. This doesn't violate the rules because they still died in the past as the result of the snap but since the snap was done by the gauntlet, it can be undone in the future via the gauntlet. That is why they could not bring anyone back who died prior to the snap ie Gamora.
So with that understood, Cap went to an alternate timeline in 1940 and lived with Peggy in that alternate timeline. The original timeline where he got frozen and Peggy went on to have kids is unchanged because that is not the timeline he traveled too as he can't travel to his original timeline. What you see at the end was that after living his life with Peggy in that alternate timeline, he used the suit to return to his original timeline in 2023. There he gives Falcon the shield which proves he lived in an alternate timeline because the shield in his original timeline was destroyed.
As for Gamora, the Gamora they pulled was again from an alternate timeline so she still existed from 2013 -2017 in the original timeline. She was simply pulled from an alternate timeline to the original timeline. The Gamora from the original timeline still did everything we saw in the previous films and then died when Thanos sacrificed her for the soul stone.
So the short of it is that any scene in the past is occurring in an alternate universe because time travel in the quantum realm takes you to an alternate reality not the past of your original reality. Thus you can do whatever you want in that alternate reality past because it affects the future of the alternate reality not the future of the original timeline.
Back to the Future's time travel works differently because they are following the Copenhagen interpretation of quantum mechanics which allows for the grandfather paradox because in that interpretation you are not traveling to alternate realities but rather traveling back to the past of your original reality. The fact that paradoxes can exist is one of the reasons that Hugh Everett came up with the many worlds interpretation of quantum mechanics which again is the prevailing theory at the moment.
The bolded doesn't jibe.
The end scene starts at the park. For cap to have used the suit to return to the timeline we are watching, he would have had to have appeared on the platform.
Having old cap arrive on the platform would have fixed the incongruity- arriving on the platform pulls him from his alternate reality after a life lived- a blink for us a life for him.
Remember- every sequence of time/timeline travel required coming through the platform, up to and including Thanos' ship.
So here is the corner they paint themselves in to- The events we watched up to rodgers blinking off of the platform were the primary timeline- the one single successful defeat of thanos and return of the stones. any other timeline or reality ended in defeat.
Rogers entering the world at any point in time in that primary timeline and appearing on the bench is an altering of history in the primary timeline... no matter how minute.... an impossibility according to banner, and a creation of a new timeline.
The single possible way for an old rodgers to re-enter the primary timeline without altering the actual past in any way is via the platform after he had disappeared.
He did not.
You can play the "parsecs" retroactive fixing all day long but each possible explanation as to how and when cap arrives to jump on the park bench creates more problems.
With the Iron clad well established "rules" of the movie, Cap would have had to arrive on the platform as an old man after he left it as a young man.
The more logical conclusion is that Banner was wrong- he was obviously out of his depth on the mechanics and Tony was on his game.
Tony wanted his daughter to exist as the individual she was, so he allows banner to spread the erroneous time travel "law" as a means to an end- undoing the effects of the snap without undoing the results of life lived for the ensuing 5 years.
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