Correct me if I'm wrong, as I'm not that familiar with it, but isn't the point of WSL to lie to applications and present itself as the Linux kernel in order for them to be able to run? If that's the case, then the fact that they have to re-implement the Linux APIs in Windows makes the case that Linux is, in fact, the core, and without it you have nothing―of course I don't believe that statement generally, but that's what their argument would actually imply.

Not like this discussion merits any importance anyway, this is just another pointless argument about semantics. Most (if not all) parts of the GNU system are just another swappable component in the "GNU/Linux" ecosystem.