The popularity of self diagnosis with gluten sensitivities does seem to be a modern thing.
I'm not saying self diagnosed allergies are nonsense (I know someone who is clearly allergic to Yellow #5), I guess I would just consider someone claiming it was all fake (rather than much of it being faddish) an ill-informed crank (and not really feel the need to push back on what they are saying).
I was a little ahead of the curve on this: I figured it out going on a strict keto diet in furtherance of my hobby of picking up heavy things (then putting them down again). I couldn't believe how good I felt, even on a diet of nothing but tuna and water. When I added things back to my diet; milk, rice, beans; all no problemo. First time I ate spaghetti, I went back to feeling like dirt. The internets informed me that other people were sensitive to wheat as well (this was around ~ 2004), and I haven't looked back since then.
I'm not saying self diagnosed allergies are nonsense (I know someone who is clearly allergic to Yellow #5), I guess I would just consider someone claiming it was all fake (rather than much of it being faddish) an ill-informed crank (and not really feel the need to push back on what they are saying).