They divided the riders into three groups: beginner, intermediate, and advanced. The only problem was that their definitions of intermediate and advanced are better descriptions of beginners. No wonder they saw no difference in the rate of injury among their groups. The study may have been almost exclusively very low mileage newbies; I'm not surprised at the high rate of injury.
Also, speaking as someone who moved to the Pacific Northwest a decade ago, new riders seem to bring many of their driving habits to the saddle and I have never seen a population of people with worse driving habits than what I see in this region. Little wonder they were injured in large numbers when they ventured outside their cages.