There is a large body of science behind author identification, with lots of academic papers, talks at hacker conventions, etc. I'm surprised you missed it. If the NSA did use authorship identification software, would it be unreasonable to believe that a privileged leaker would have access to such a tool? I believe that it is more likely than not a joke, but it has the same effect as a right handed person signing their name with their left hand... unless they have a habit of making "In Soviet Russia..." jokes - then they're screwed.