I think that study is not valid; not from what I read in that article.
I'll have to see the original source though, instead of going by a news article, but it seems that they just looked at the raw number of deaths, without relating it to number of registered motorcycles or kilometres driven.
The speculations are interesting though.
The statistics I am looking at suggest no change in the averages since the 1970s. We're still having the same crashes as we were back then. We die less than before in the lower speed crashes, but we are having more high-speed crashes (most of which are lethal), so the overall averages haven't changed.
Instead of looking at the number of deaths, they should look at the number of deaths per motorcycle kilometer, and at least break it down between urban and non urban areas, but hey, that's what I'm here for, right?