The GNU userland is copied verbatim from your GNU/Linux distribution. Microsoft doesn't need to maintain these utilities. What Microsoft is developing is really a compatibility layer for running ELF binaries and handling all the system calls expected by Linux programs. So it's accurate to say Windows Subsystem for Linux. I haven't tried but I bet a GNU-free Linux system like Busybox will run just fine.
It emulates a Linux kernel, by handling Linux syscalls - it doesn't care much about what's in the userspace. The GNU tools are not required, they are just present since the Linux distros available use them. You can, of course, run busybox if you please. It might be buggy in some fashion but it will almost certainly work.
I'm just waiting for the rms rant demanding that it be called GNU/Windows. :)
Then again, I guess rms cares much more about the non-free nature of Windows...
WSL's kinda the mirror-universe Wine, but not quite - AIUI, it doesn't translate Linux syscalls into Windows ones, it's an alternative Windows kernel ABI that bypasses the normal one entirely.
That makes a big difference when it comes to the implementation of syscalls that have no remotely close analogue in regular Windows. fork() and exec() spring to mind - the hoops Cygwin had to jump through to fake those[0] are pretty eye-watering, but for WSL Microsoft added new kernel functionality to support them directly.