[menage]

Celiac disease and Non-Celiac Gluten Sensitivity are very different beasts in terms of their effects. The latter is more likely to triggered in the presence of (as you say) non-trivial amounts of gluten; Celiac disease is an auto-immune disease that, while not normally at the level of anaphylactic shock, can have very unpleasant effects in tiny amounts.

And even within Celiac disease there are a lot of variations. My mother has been diagnosed as Celiac for 50 years (back before most doctors had heard of it, and she almost died from malnutrition pre-diagnosis). She obviously avoids anything that has any mention of wheat, or has been cooked in the same oil as glutenous food; she generally avoids anything that mentions "possible cross-contamination" on the label but doesn't have to be a total stickler. I don't think she's had a serious attack in many years. A friend of mine was diagnosed as Celiac maybe 10 years ago, and is incredibly sensitive - despite his best efforts he seems to end up with horrible symptoms every couple of months or so just through tiny amounts of contamination.

The FDA limit for claiming something is "gluten-free" is 20 ppm; I believe that level exists partly because it's very hard to detect anything less than that anyway, but it also fits in well with what most Celiac sufferers can tolerate.