Ubuntu and its derivatives are included.

Yes. But Red Hat, Fedora, openSUSE, SLE, Arch, Alpine, Slackware, Gentoo, NixOS are not. I have a hard time to think these represent less than 10 percent all together

Is Ubuntu still Debian based? What's the criterion for that anyway? Just using debs?

Ubuntu releases are, effectively, snapshots of Debian sid every 6 months. That's a pretty strong basis.

I wouldn't call Ubuntu a flavor of Debian as they have so big differences, but ultimately it's still mostly Debian. You can see a numerical comparison of packages here: http://qa.ubuntuwire.com/mdt/all.html

Yes, Ubuntu is still Debian based with just a little added on top.