It's worth noting that subsystems are an old Windows concept dating back to the original Windows NT in 1993. The win32 API itself is a subsystem of the underlying NT system. And the WoW64 subsystem allows 32-bit Windows apps to run on 64-bit Windows. There used to be POSIX and OS/2 subsystems, as well.

I believe the "Windows Subsystem for x" nomenclature is a departure from the way MS previously described subsystems. That doesn't mean it is wrong, though. Just a bit inconsistent.