It's a bit strange, because it seems Docker points to Wikipedia about this sentence, but Wikipedia article says: "that is composed entirely of free and open-source software, most of which is under the GNU General Public License..." (emphasis is mine).

I'm not sure why Docker changes "entirely" for "primarily"...

But going to the point, about statistics on license usage in the Debian archive, I've found this blogpost from @Stefano Zacchiroli which references a talk @johns gave at FOSDEM 2012. Both the talk and the blog post give interesting data, and discuss about methodologies for counting the license usage. HTH!