That's not true at all, and I'm tired of hearing it claimed. The format of The Marvels was ideal for bringing an audience up to speed without having seen the shows, because the lead characters were meeting for the first time or at least for the first time in decades, so they all had to explain their backstories to each other, which was the perfect way to let the audience learn about them in the process.
It doesn't matter if that's actually true, that's the impression the people who didn't watch the movie got, and became part of the post-release CW.
Frankly, given the extensive re-edits the movie clearly had (with clunky exposition scenes plopped in), it's likely the original cut was a bit more reliant on prior knowledge of those properties as well.
I'm pretty sure he is, the post credits scene showed him looking into the camera with an orange sparkly circle portal reflected in his eyes.
Technically this is a deleted scene that Ryan Reynolds dropped post-release. I believe it's show in really low-res cut-up section of this scene on one of the TVA screens, but I'm not sure if enough made it into the movie to be officially canon. Though of course canonicity means little given the multiverse.
Regardless, D&W implied he'd be rescued by the TVA (along with Blade and Elektra, presuming they survived too). That would seemingly put him on a different "team" than the other X-Men that we're going to get. Or, at the very least, the Beast introduced at the end of The Marvels is from a different universe than the Gambit introduced in D&W. Kinda has to be. I mean, I know Gambit was technically in X-Men Origins: Wolverine, but it's pretty heavily implied that the Gambit we met in D&W is from a pruned timeline.
I'd love if we get introduced to Gambit as Mobius's new partner or something in Doomsday (really seems like the TVA needs to be involved, and we have to see Loki somehow), though that would kind of require a lot of other characters being dragged in.
Sadie Sink for sure will be introduced IMO. She has got to be playing Jean Grey I would imagine.
Except she's reportedly being introduced in Spider-Man 4, which is a weird place for Jean Gray to be.
Of course, Spider-Man 4 is going to be released between Doomsday and Secret Wars, so for all we know it's all on Battle World or something.