> celiac disease actually exists, but is very rare

I wouldn't call a disease that affects around 1% of the population "very rare".

It's true that there's a gluten-avoidance fad that has drawn in lots of people with no medical justification, so that not everyone who demands gluten-free food is necessarily a celiac sufferer. "Gluten intolerance" seems to be a fashionable ailment these days, with perhaps varying degrees of validity (and potential confusion with other issues that may have similar symptoms), but sources suggest that around 3 million Americans, for example, really do have celiac disease.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3496881/ http://www.uchospitals.edu/pdf/uch_007937.pdf