Dietary fiber is not a micronutrient. In fact, it provides exactly zero nutrition because we cannot digest it. In fact, that's the sole reason it's claimed to be beneficial.

And what kind of vitamin or mineral is dietary fiber, hm?

Thirty years ago, I knew exactly nothing about diet or health, let alone gluten. I was 18, never heard of gluten or celiac or anything like that. I find it very unlikely that anybody else would have known anything about gluten back then either. A thirty-year follow-up of ignorance is meaningless.

If we want to hypothesize an alternative explanation, it's the gluten-free vs whole grains. There's tons more whole-anything than gluten-free crap. The idea of whole-anything being more healthy than refined is much much stronger and prevalent than any idea about gluten. Ergo, any effect can be explained by the tendency to adopt a multitude of genuinely better choices non-diet when choosing whole-anything in diet, vs when choosing gluten-free.

The 13 percent figure is relative, not absolute. The absolute figure is closer to 0.1%, a quantity that is easily explained by even the most benign mistake.