Think it's fair to say Endgame confirmed the existence of a multiverse when the Ancient One described the branching timelines. That said, my money is on Mysterio just lying about coming from another universe. Another alternative is that he's trying to reinvent himself as a hero in this universe. Regardless, the monster attacks are of his making. He's probably Loki. Okay, that's going too far.
The Ancient One specifically described the multiverse back in Doctor Strange, she just didn't explicitly say it included alternate Earths. I'm of the opinion that Mysterio is both a liar and from another Earth (also seen some people bouncing the idea that the Fury we're seeing is an imposter, maybe even a Skrull). I'm not so sure on Mysterio directly causing the elemental attacks though. Recall how in Endgame, the Ancient One said that removing Infinity Stones from a universe will open that universe up to all kinds of dimensional trouble. The "prime" timeline no longer has
any Infinity Stones. They were all placed back in their proper times and thus their timelines ended when Thanos snapped them to pieces in 2019. These things could be coming in through holes in spacetime all on their own.
If they're really committing to this and Far from Home is serving as the coda for phase 3/prologue for phase 4, I'm thinking that phase 4 will basically be the Marvel version of Crisis on Infinite Earths. Stuff is going to get
weird.