Since I've fought all my life with sleep terrors and other disturbances, I have deep empathy to people who experience frightening mental imagery. It may not be real, but it can be really distressing.
I just mean that there are different kinds of "experiencers", not all just want to believe in UFOs and be in the club.
I don't think Mack was totally off with that one.
He was, however, wrong to assume any of that was real in a physical way and also wrong to consider hypnotic regression a reliable method for treating memories and trauma.