Only Facebook commenters have "enforced" real names, in that their Facebook name is used. Signing in with a Disqus account or Twitter uses the names that a user chose on that service, which typically is not a real name but some sort of pseudonym.

So while it's true that it's basiaclly comparing Facebook-identified commenters to other commenters, only Facebook-identified commenters are required (assuming they sign in with their actual account) to have a real name.