Interestingly, his books describe very psychedelic type experiences, but he apparently didn't do psychedelics much. There's a reference to him trying LSD once and hating it. His drug of choice seemed to be amphetamines.
Ultimately it's impossible to diagnose post mortem what exactly was going on in his head.
Just because PKD said he hated LSD doesn't mean it didn't influence his writing. The peaks of many of his best works seem like bad acid trips, though the paranoia that can come from heavy amphetamine and other drug abuse could certainly have had an influence as well.
Anyway, I'm not sure how trustworthy PKD's accounts of his own drug use are.
PKD wrote a lot of great books really quickly by relying on amphetamines, heavily. This had a major contribution to the far out mental state he was in towards the end of his life, and it's actually quite sad.
I say this as a huge PKD fan. Ubik and A Scanner Darkly both had a profound impact on me.
Lots of people have written very trippy things without having done psychedelics at all.
Here’s something I was just learning about in a podcast recently — 2000 years old abd similar to the kind writing that pkd was doing after his breakdown.
While I agree that very strange and possibly "psychedelic" things can be written without using psychedelics or other means of attaining altered states of consciousness, particularly if one is mentally ill, I'm not convinced the 2000 year old works the article you link to discusses weren't themselves influenced by access to some altered state of consciousness, perhaps through psychedelics.
Psychedelics and other means of altering consciousness have been used throughout human (and possibly even pre-human) history. In reference to the link you cited, there has been evidence that cannabis, which can have psychedelic effects at high doses, was used in ritual contexts in biblical times in ancient Israel. Here is a recent article discussing some new evidence of this:
Ultimately it's impossible to diagnose post mortem what exactly was going on in his head.