Well, the rumors are mostly going on the fact that
A) There's that video floating around of him playing with Rudess after some event
B) He's really good and could learn their discography in a matter of weeks as well as having a style that would fit well in the band
There's nothing that would really suggest he was asked to audition/was accepted other than that. Mangini canceling all his lessons for the next year is very interesting though.
Of course, there's a chance that someone else that nobody expected will be announce and everyone's heads will explode