Spiderman far from home****

Spiderman no way home isn't even out yet

Mysterio was accidentally right about the multiverse.

The answer is that multiverse always existed.

The tricky thing about the idea is that an action outside of the timeline affects it as a whole. While "Who He Remains" was alive, the multiverse did not exist. But his death by Sylvie affects the entire timeline, and from a timeline perspective, the multiverse has always existed.

The Loki series takes place outside of the timeline, that's why the characters can go to any point on the timeline to correct or prune it. Loki's series isn't part of any Spider Man timeline.