Star Trek has multiple forms of time travel. Mostly they go with the "everything snaps back but we still remember", sometimes they go with "everything snaps back, nobody remembers but things go differently by chance". One episode has "we've done all this so many times that it's giving us MAGICAL PSYCHIC DEJA VU".
The problem with snap-back-remember is that in order to remember the previous timeline/changes, the previous timeline has to have happened (eg "Timeless"). This must mean that it takes place in an alternate reality (because the main reality simply doesn't happen that way, so by a process of elimination...). This is sometimes referred to as a "pogo paradox" - the original timeline happened (Zephram Cochrane makes his warp flight), but then is changed (Earth is assimilated by the Borg before Cochrane's flight can occur), but then is restored. The differences aren't important (though as with Year of Hell it's possible to have a pogo paradox where there are no or very few differences to the timeline at the point of resumption) but - crucially - the post-change timeline relies on the pre-change and the changed timelines to have happened, ie, it must begin at a point after each of them has happened.
Contrast this with Back to the Future where changes are visible in real-time (Marty and his family fade out of the photograph) because of paradoxical changes to the sole timeline: but this is still a pogo paradox. If Marty fucks his mom, she never hooks up with George and Marty can never be born, but if Marty is never born, he cannot go back in time and fuck his mom, so she hooks up with George because that's already what happened.
With the MCU predestination is explicitly real - it just can't be changed. When Rogers goes back in time, he remains in the MCU main universe. That means he must *always* have been there in the background and even if his presence caused a branching timeline, that branch is always the MCU we've known.