My understanding is there's always been some idea of a comedy based ride in development, prior to Tonight Show it was SNL. Seems like there's been plans for something NYC related in that area of the park. When I first heard of this project before Diagon opened it was "30 Rock", which turned out not to be the show but the exterior. Sounds like they went with Tonight Show based on ratings and Fallon's YouTube appeal.
If you skip to 2mins 50secs, he says he pitched the ride himself. This could be all show though:
Jimmy Fallon on How He Scored His Own Ride at Universal Orlando