Radical relativism gets you nowhere. Ultimately, EVERYTHING is an "assumption" or "guess". That's how it works. We never have ALL the information, and even if we did, we would have to question our perception of it.
We go with the most reasonable one that has the best evidence supporting it and discard the others. so I am going to discard unfounded claims about the mystical nature of experiences created by drugs. Is there a possibility I am wrong? Yes. There always is.
And it's a tiny enough chance to declare anyone believing in that mysticism to be utterly confused just as I would declare anyone believing in homeopathy to be utterly confused.