If the source was scientific (controlled lab experiments) then that should be taken at face value. If the source was anonymous, then large number of multiple, identical accounts should be separated. It would particularly be logical to at least establish who/waht the clearly evil entities/acts are.
Supernatural (the book) is an all out attack on the "imagination" explanation. I have no 'feelings' one way or another - just trying to rely on my own intelligence. In order to accept imagination as the source, which I'm perfectly willing to do, it seems that we would have to explain why a modern person would come up with same specific images and acts not found anywhere in our material world, like someone from another time/continent/background did. Odds are unacceptably astronomical.
We can also argue in agreement that we don't necessarily have to rush to conclude that images are "real". We can say that those specific images are "hard wired" into our brains/DNA but then both the "hard wired" explanation as well as the brain tuning into another "frequency" and acting as a receiver explanation seem to win out over the simpler "imagination" explanation.