It's delusional in the same way that someone else telling me they had a religious experience is delusional.

I cannot prove [to myself!] after the fact that what I experienced under the drug was real. I obviously believed it at the moment I was under the effect of the drug, but afterwards I couldn't recall exactly why I believed that. In other words, there was no way to prove or explain it. So it's delusional -- an event is supposed to have happened but there is no proof of it.

There's simply no reason for me (now) to believe that what I experienced under the drug was real.

(As I said above, I still think LSD should be legal so we can scientifically look at reducing harms from people taking it)

Well when you're going at it from that angle, how can you prove to yourself that anything you experience is real? There are centuries of thought surrounding epistemology, I don't see why problems of how and to what degree we know what we know should apply to drug induced experiences and not regular experiences.

But then again, I don't know what you experienced. The most profound insights I had make perfect sense to me now, and they mesh with current scientific understanding as far as I can tell, so I have no reason to doubt them.