I'm confused (nothing new) but reading this it does NOT sound like it was a study where they tested to see the effects of weight on a horse. There are not testing mechanisms for muscle or joint wear, or other testing parameters to monitor how the horse is fairing under the stress of the rider's weight.
Rather they had pre-set standards (under 10% ideal, 10-15% satisfactory, over 15% detrimental) with no indication on where they got those numbers. Then they took a random group and the "study" consisted of figuring out who fell in what range for their current mounts. Nothing more than a numbers game and a total BS study.
Pretty much what I said, no mention of where they came up with the numbers. Even if they did it would still be total bull.
Historically its wrong, at tevis its wrong, thousands of years of horse riding shows its wrong. But oh well I guess it gives skinny little girls that spend more time gossiping in the barn and picking out what color breeches are proper than actually riding something else to feel superior about.