This theory is stupid. It’s just taking what is already symbolically there in the movie, and pretending it’s literal. Also, if Ferris isn’t real, why is there a B plot about his (also not real?) sister?
I mean. Ferris is an unrealistic character grounded by several realistic characters, he’s the kind of person everyone wishes they could be: Someone who doesn’t let life pass them by. And it’s about Cameron, a more everyman-type person, learning from his exaggeratedly more has-it-all-figured-out friend. And everyone feels like they know someone like that, because “having it all figured out” is relative; you feel lost and your friends seem to have it all together, but the reverse is also true for them. In that sense Ferris is the fantasy element of the movie, but in the reality of the film, he’s a flesh-and-blood human being.