I don't want to argue with your overall point at all, because I assume that whatever this sentence specifically means to you is true for you - but given that there is no actually objective definition of what a healthy diet is - it's hard to agree with the statement in general. I would feel like I was eating unhealthily if I had a can of coke every day, but that doesn't actually mean it's unhealthy. It probably depends on everything else I'm eating, how much I'm burning off, maybe even something as fine-grained as how fast I drink it. You would almost definitely feel unhealthy if you were eating bread every day, but as long as it was whole-wheat and had some good sandwich fixins in it, I'd feel pretty good about that.
Anyway, sorry if I'm being pedantic, but I feel like people toss around things like "healthy diet" as if we're all supposed to know and agree what that means.
Eating bread every day is actually pretty common in some cultures and people don't even stop to think it's unhealthy because it's so commonplace, so even that varies.
(I have a sibling with celiac disease).
I've noticed things that never had glueten in it, labeled "glueten-free" after the craze a few years ago.
Most of the "health fad" people you're talking about would probably self-categorize themselves as NCGS, but that doesn't mean that everybody who is NCGS is part of a health fad. Double-blind "challenge" studies where people are unaware if what they are eating contains wheat/gluten have confirmed that for some people this is definitely real and not a made up fad.
I'm not denying the existence of people going gluten free for fad or orthorexia reasons, but be careful when you lump everyone who is gluten free for non-celiac reasons into that category.