Dumb question: Why did they name this "Subsystem for Linux"? (Old name was Interix, subsystem for UNIX-based applications, or "SUA".) Does marketing/users believe there is a distinction between Linux as a kernel and GNU as a source for a userland? Irrelevant? Curious what others think.
The big difference is that it isn't just POSIX compatible, it is a full out "bug-for-bug" emulation of the Linux kernel specifically with an eye towards binary compatibility. Out of the box, the userland here isn't just GNU, it is entirely the same binaries as shipped by Canonical in Ubuntu.
Old Interix required retargeting and recompiling binaries, similar to how you would need to recompile to target the differences between Linux and BSD. WSL runs Linux binaries without the need to retarget or recompile specifically for Windows.
This also means that the long tail ecosystem of Ubuntu PPAs and custom binary download sites all mostly work, too.
There's still enough GNU there that you could call it that if that amuses you to do so. (Though I bet RMS will happily throw a fit in your direction if you do. ;)
The old stuff provided source-level compatibility with UNIX via posix compliance.
The Subsystem for Linux is essentially an implementation of the Linux ABI, which is distinct from UNIX and provides true binary compatibility.
The ironic thing is that WSL itself is deliberately not Linux; the only similarity is the user-space ABI.
Unfortunately, I've met very few developers (admittedly interacting mostly with students) who have any understanding of what is Linux vs. GNU utilities vs. UNIX, vs. Ubuntu. "macOS is basically Linux right? Its because of the Terminal". screams
