The "true" GNU Operating System is essentially every distro that's fully free software - to the point where you're excluding things needed for basic functionality like firmware blobs, with the proposed solution of "just use hardware that doesn't need that software". Only about half a dozen distros go the mile to get that stamp of approval, and most of them are forks.

You might think that Debian fits that criteria, but offering to enable the nonfree repo (and likely the nonfree repo itself existing) disqualifies it as an endorsed GNU system. That's the comparison.