First, the authors and books were not chosen randomly and the sample size is tiny, so the results are meaningless, although I do respect the effort that was put into this. The author is up-front and honest about this in the beginning, but unfortunately proceeds to draw unwarranted generalizations based on his study.
Second, even if a similar, but much larger, study were conducted on randomly selected authors and books, the results would still conflate the vocabulary size in common usage with the vocabulary size of single authors. Hence, what the article is attempting to study is not the same thing as the title of this post.