spoilers:end of spoilers
Yeah I don't buy it, it's Mysterio we're talking about, the master of illusions and deceptions, so I wouldn't be surprised if he's lying about being from another universe. My guess is he's working with the elemental creatures or those things are just illusions he created to fool everyone for whatever reason.
Anyways that's a nice trailer, I hope the movie will give us a nice ending to the phase 3.