You can't take a binary compiled for Linux and plop on a ChromeOS installation and expect it to run. It works currently because ChromeOS runs Linux applications in a lightweight VM with a Debian environment.
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.
Huh? I do that all the time (run Alpine compiled exes on other distros). I use an Alpine Docker image to build statically linked exes (using MUSL as libc) which are distro agnostic since they don't depend on glibc. Works just fine (for command line tools at least).
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.
New syscalls may be unavailable on distributions that use old kernels. It is probably possible to tell whatever libc you use to avoid fancy syscalls, but still there may be problems. Note that the other way should work (from old kernel to new)
With that logic, Why not count BSD as one? Linux is the kernel, but Debian has Gnu/HURD also.
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.
The BSDs have diverged significantly since then and not just in userland. Unlike Linux distros they do not all have the same kernel. There are of course common parts in their kernels, many of which date back to Unix, but there are also big differences between all of them.
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.
