Not much analysis in that article, but it is easy to see the possible problems with this research. Since B+C only records data on the largest animals, little is known about the overall size of antlers in the herd. B+C animals are always a very small subset of the population, and perhaps with today's emphasis on records more animals that "just" make the book minimum are being submitted, skewing the average size of the entered animals. Quite a few people don't enter their trophies as well, so the small subset of animals that are recorded by B+C show little about the overall number of trophy size animals present in the herd.
I see an article based on data analysis of a presumptive PhD candidate who needed some conclusions to justify granting his degree. A 1% change in average antler size doesn't seem like much, and certainly not enough to recommend changes to our seasons.