Note that Flatpak is not a distribution, but a packaging format.
BoilingSteam’s article in which their thoughts and reflections are written can be found here.
While it’s technically not labeled, the blue-colored columns found right below openSUSE belong to Garuda; as can be seen here (from an earlier iteration of the graph).
Technically, Linux Mint also has their Debian Edition. But, the vast majority of its users should be using the one based directly on Ubuntu.