What they experienced is an inward look at something they don't like and resulting denial that it was real. They're not ready to look inward because there's something there they still can't handle to see. Doesn't mean you can't have a "bad trip," but typically what makes a trip "bad" are the thought spirals that take you to places your brain clearly wants to go, that you spend your life preventing it.

People who say "It messed me up long term" in some way by LSD (not just "I had a bad trip") are really demonstrating a concrete wall around their sense of self (ego). Anything threatening to the ego is, to them, damaging. Bad trips for those who are capable of inward reflection turn into revelations, so long as you're in a safe space and the dose is not too high, as with anything.

If you really WANT to get something out of LSD, though, you have to go into it with that intent and with the understanding that you might see something about yourself that you don't like, and that's ok.