And considering everything that people like Peter Parker and Nick Fury have been through in the last five years, the idea that the multiverse is a thing would certainly not be the craziest thing to happen recently.
The idea that he’s using the multiverse as a cover for his plan isn’t that much of a step to take.
It would explain to everybody else where these creatures came from and also keep Nick Fury and others from taking too close a look at Beck himself if they think he’s from another world.