I've heard that it could be cross-contamination from the bags used to carry the coffee. As in, the bags may have carried grains previously, and thus been contaminated with gluten/flour while carrying the coffee beans, contaminating the coffee beans. It could also potentially be some other part of the processing chain.
It's not clear to me, though, whether it's cross-contamination or cross-reactivity. I thought I had a cross-reactivity to chocolate, but then I found some brands of chocolate that are safe for me, leading me to believe that my reaction to most brands of chocolate was due to cross-contamination. I'm guessing the same might be true for coffee. On the other hand, there are a lot of compounds in coffee, so it could be cross-reactivity to one of them. I'd love to hear if anyone has had the lab testing you've linked to. I'd love to do it, myself, but $280 is steep!
As for the popularity in the paleo community: I think the trace amounts of gluten won't bother most people unless they're really sensitive celiacs. I have friends with celiac disease who don't need to be anywhere near as careful of what they eat as I do, and yet feel great and show up clean on blood tests and biopsy. Many celiacs are just fine eating below 200ppm or 20ppm gluten. So even if there is like 2ppm of gluten in coffee, I think most paleo people don't need to sweat it.