Lots of people self-diagnose as coeliac because they know they have real symptoms from eating wheat, and there are a lot of people who claim to be gluten intolerant because they're on a fad diet rather than because they actually have symptoms. That means that there's an impression that non-coeliac gluten intolerance isn't real, so some people who have it know they've got a real condition, so assume they have the thing no one's claiming is fake.