If you so desired, you can enable Linux support, which will run a Linux container just like WSL on Windows, for the Crostini environment.
If that counts, Windows, IBM mainframe/micros, and Unisys mainframe/micros are also Linux.
Given that, I wouldn't count ChromeOS as Linux, because if we do, by that logic, we'd have to count Windows as Linux due to WSL.
But you can ( with some effort ) make it run in ChromeOS.
I often compile things inside Alpine containers statically and run them on my local machine, and plenty of times I've downloaded deb packages, extracted them on a raspberry pi running Alpine and it's ran fine (assuming glibc compatibility is set up.
"Different enough", I guess is the mark.
E.g. "Ubuntu is a reskin of Debian with spyware"
Linux in that list is flavour, but that is also the same with BSD. BSD is the flavour and Free, Net, Open all come from BSD 4.3. And if we really want to push it, it also includes MacOS 10.
I was also surprised to see Sailfish OS, Meego and Maemo listed separate from Linux, but my guess would be that the list comes from the build system of curl. Everything that is its own build target is listed there.