I mean, it is Suth's interpretation, doesn't have to condone with the wiki's interpretation. The exact place where the main MCU timeline sits, according to the wiki's timeline, is based on parsimony rather than personal head-canon, but when the window is open for interpretation, its fine to think the timeline where Gert dies is the one where Endgame and the like happens.
I do think, however, that the timeline where Gert does not die is the one where the main MCU timeline is set. You see, the time-travelled runaways from the future pretty much fade away from existence once their futures are changed, one could compare this to a timeline pruning (or if you want to get more free in the interpretation, maybe they returned to their timelines, alive, ala NWH multiverse characters). But if their timeline is pruned, it blocks the likelihood of the Gert death timeline being the one where the main MCU timeline is set, unless the main MCU timeline branches out before 2028, allowing it to continue on from the Gert dies timeline. Its confusing, though.
My take about what the Runaways are doing after the events of S3 is that they tried their best at returning to normal teenager/young adult lives. They don't seem to be the type of people to stick around as well-estabilished superheroes, at least in the context of the show, they just wanted their problems fixed, and if that involved saving the world, that's fine. As mentioned by other people, the Runaways all did survived the Snap, as the time jump in Runaways S3 transports the Runaways to a post-Snap world (even though it isn't explicitly shown as such in the show, but we can infer it through timeline calculations). That said, the Runaways will grow up and lead their own lives. For instance, by 2024, all the Runaways (I think) will be on their 20s, and will probably be in college or something, and some retain powers and will probably want to try and use them, be it for good or evil. Imagine Nico Minoru joining the ranks of the Sorcerers. Karolina and Molly are still enhanced individuals, maybe they will be drived to make a difference in the world if they feel compelled. Anything can happen.