There used to be a rule in Marvel time travel that you can't change the past, you just create alternate timelines. That rule is constantly ignored in modern Marvel, and it being acknowledged is the exception, not the rule. Characters constantly act like changing the past or knowing too much about the future will destroy the universe even though before they knew it wasn't the case. Why do you think writers don't use this rule anymore?
Because it was a dumb rule.
Mark Gruenwald in particular was a big proponent of this concept, because he liked things to be orderly, and he spent a lot of years hammering nails into the idea that this was how time-travel worked in the MU—invalidating several earlier stories in order to do so in some cases.
I on the other hand have now spend an equal or greater number of years prying all of those nails back out again. Because if any time-travel story just results in an alternate timeline, then no time-travel story has any stakes or any drama. It’s neat and clean, yes, but also bloodless. Time-travel should be dangerous, it should be risky, and there should be consequences, potential or otherwise. At least that’s my feeling.
So, my assumption is that there are several different methods of time-travel that exist within the MU. Some of them create alternate timelines, and some of them do not.
I thought it was interesting that Marvel's more fluid approach was actually a result of opposing philosophies being promoted by different editors. It certainly explains why so many recent time travel storylines seem to run away from the multiverse concept.