Well, how do we distinguish between GNU systems and BSDs? And FSF doesn't seem to recognize Alpine as a GNU system either (the #justlinux section of their FAQ). Same goes to the Alpine devs: they recognize Debian and such as GNU+Linux but think that Alpine isn't GNU+Linux. IMHO, something is from a different operating system family when it requires compatibility layers to launch software from other types of OSs (like with Windows software requiring Wine).

Have other OS families ever switched C libraries tho? Coreutils?