If "The Snap" did in fact tear a hole in the universe and generate the multiverse that Phase IV is gonna be moving into, that's a really smooth way of being able to integrate all of the fun extras without needing to worry about them retroactively impacting the continuity of the MCU. That being said, some folks feel like this is a red herring, because well... –
it's fucking Mysterio.
Here's what makes me think it's legit:
- First off: Mysterio, Nick Fury, & Spider-Man meeting as trans-universal travelers is basically the plot of the comic "Spider-Men"
- Nick Fury is the one who says it in the trailer, so that generally means that he's done some of his own digging on whether or not the claim is bullshit, rather than if it was just something that Mysterio was claiming outright.
- If Disney wants Spider-Man to fill the gap that Iron Man left behind at the end of Endgame like the trailer suggests (not to mention the fact that that's Spidey is basically guaranteed box office gold), that means that Disney & Marvel are definitely wanting to play nice with Sony, since the web-slinger is still theirs.
- Phil Lord & Chris Miller (who made Spider-Verse) have a five year agreement to make a series for the Marvel TV Universe for Sony, and animation's always been where DC has solidly 1-upped Marvel, but that could give them a really big step into that space.
- Both Spider-Verse and Venom did absurdly well at the box office, so letting both of those sandboxes exist on their own, but giving a path to them to play together with more MCU stuff through Spider-Man & the Spider-Verse essentially already writes itself.
- On top of everything, this also gives Disney & Marvel a way to move in and adopt all of the things that they just obtained from Fox in a smooth way that doesn't disrupt any of what they currently had planned out.
- They can use things like the Black Widow film as easy entry points with a familiar character to show us what things are like in a universe where they're still around, but things are... different than we'd expected.
tl;dr – it's something that mirrors the comics, it's straightforward from a storytelling standpoint, and it's damned good for business.