It is such a human universal (at least until a person theoretically develops the wisdom to know better) to deny, belittle, or disbelieve any experience outside their own.
When people do that, it's a sign of their own immaturity. I am learning to be compassionate about it when I encounter it in someone else, because I can look back at myself, and not too long ago, I too tended to snort or do an inward eye-roll at anything at all outside my own personal experience, as if my experience were universal and all-encompassing, so that anything outside of it, had to be false.
I did it with food allergies (til I became the food allergy mom!) and with countless other things in my growing-up years (and sadly, for me, that went into my 20's and even 30's!)
Strange as it sounds on examination, even people who love and know you best, until and unless they deepen in wisdom enough to get past it, seem to find it easier to believe suddenly that you are a liar or hallucinating, than to believe that they themselves could simply be mistaken about an experience they haven't had.
Isn't that a hoot, when you put it that way? That might also be a good reply to the person doing it : "Have you considered that you normally believe in my sanity and truthfulness, but now find it easier to believe I am either lying or hallucinating, than it is to admit that you have trouble believing in what you yourself have not experienced?"
And the funny thing is, we're all so prone to it. It's like finding out that we're all basically very little different from the monkey who can't pull his hand out of the hole, because he refuses to let go of the peanuts, ( or the ignorant person who, when shown a photo of a giraffe, declares, "there ain't any such animal!" and walks away unfazed, certain the photo was a fake) and that very few of us mature to a point where we can overcome these habits. I'm pretty sure I still haven't overcome them, even though I am now aware they exist. I just try to recognize the urge when it comes on, and go, "Oh, yes, I'm doing that again!" and then try to think differently.