It's a bit more than just some candy, there's substantial glue on both the Linux/Windows sides to get Plan9, WSLG, and the other components to work.

That said, the kernel they distribute is open source and you're not limited to just the distros they're working with directly. There are a number of third party (e.g. there's no Arch from Arch or Microsoft, but there's a completely compatible third party package that gives you Arch in WSL2)