Which raises the question: what if Chris never saw the movie 'Who Framed Roger Rabbit' or basically any media that involves going into a cartoon/animated world (I.E. Mary Poppins when they jump into the street art)?
Nah, even when not exposed to stuff like that, kids still have imaginary friends and whatnot. My personal and entirely unscientific explanation is that religious feelings in adults come from that same part of the brain, only tempered with rationality to avoid scenarios that defy logical explanations.
This was easier thousands of years ago when there was no good explanation for say, a bolt of lightning -- it was perfectly plausible to attribute it to an angry god. The appeal of religion remains today even though they have to take more abstract interpretations of it (God caused the electromagnetic force to exist, and now lightning bolts happen because of that).
The imaginary friend, dimensional merge, etc. is just that same feeling unfettered by the need for rationality. Learning to control it is a time-honored rite of passage for children as they learn to stop playing make-believe. Even in times when most people believed in ghosts (and they still do in much of the world), there's a certain understanding of the line between make-believe and true-and-honest ghosts.
When I was very little I talked to imaginary cartoon friends before I saw media like Roger Rabbit. In fact, I found Roger Rabbit odd and amusing because of how the toons could actually physically interact with the world instead of just hanging out there.
Come to think of it, Chris doesn't even reference Roger Rabbit that much. Most of his toontown shit seems to come from Rick & Morty, media he consumed later in his life. Just further evidence that Chris' brain never developed beyond make-believe.
EDIT: Now that I think of it, I guess I'm a bit wrong about Roger Rabbit. Old Chris definitely formed the seeds of toontown beliefs from Roger Rabbit, using it in explaining his beliefs to Alec Benson Leary. The Rick and Morty stuff simply took more prominence later via the Idea Guys. That said, I still stand by the make-believe explanation.