I will see in the future lots of tutorials, guides, software, company support, etc covering not just your regular Linux but also WSL, since it's much easier to access rather than installing yet another Linux distro directly on hardware just to try Linux software.
My point was that you can't claim the 100 million+ Windows userbase as being the largest Linux distribution, because almost none of those people (proportionally) are actually using WSL.
I don't think we have the usage numbers for WSL. I don't think we have the usage numbers for Linux desktop either. I personally would be surprised if WSL was the largest Linux distro, but either way it's not a claim one can make because we don't have the numbers.
It has achieved Linux on the Windows desktop without dual booting, all unified, download whatever distro you want.