Hmm. I don't know too much if any about mysticism either. You're view is as good as mine.
Well, I think what these particular mystics were doing, Carlita, is assuming that because something (e.g. their "experience of deity") felt real that deity was real. However, it's the brain, and not reality itself, that creates that feeling something is real. So, the mere feeling something is real happens to be an unreliable guide to whether or not it really is real.