everything you need to experience whatever you want is already in your head
No one's special, and schizophrenia has a bit more going on than hallucinatory trains. The list of symptoms unrelated to that are vast with a wide amount of variation from person to person even within their multiple subtypes.
Many disorders can make meditation become next to impossible, and I am among those. Their center tends to be found elsewhere, usually something active enough to both focus and exhaust at the same time.
I'd be more likely to believe it if multiple schizophrenics could witness the same things simultaneously without having to have one sell the idea to the other for it to happen. I'd also believe it if their visions didn't reflect their own minds so strongly.
It's not an alternate universe, it's a manifestation of their own inner creativity overlaying the physical world. It's basically like dreaming with your eyes open even down to the sorts of absurd behaviors that can occur while in such a state. Even word structure and keeping a straight train of thought can fall apart during such cases in a surprisingly similar way, and by the end of it much of it can be blacked out much like that of waking from a deep slumber. At times it's energetic and dizzying while during others it's dreamy, fuzzy, glowy, out of body yet still trapped inside a form that feels almost foreign, fungal in nature.
At most you could stretch it to being a portal to the land of dreams, the realm Morpheus was shown to be in charge of in the Sandman series, and even that goes into fantastical hogwash and magical thinking risks.