What about Ruby?
I'm not saying that this metric measures language popularity, but that language popularity seems to have a big influence on it. I may be wrong, but that's what caught my eye when I read the article. There are plenty of other factors that aren't accounted for, some of which have been pointed out by other commenters.
Considering this I'm not surprised at all to see that Java, Python and C++ repositories have a higher "bug density" than Clojure, Go or Erlang ones.