The FDA has the ppm for Gluten-free items as under 20 ppm. From some experiments I've heard, Clarity-Ferm will produce beer under that threshold, but it's not guaranteed. If you label a product as gluten-free, you would have to guarantee that every batch is under the threshold.
Unless you're really trying to lock down the gluten-free niche, I'd imagine you wouldn't want to have to test every batch. And if I were a brewery the size of Stone, I'd hate to have to dump a batch for something that tests over the limit. It's much safer to market your product as "gluten reduced".