Earlier quoted context omitted.
Next? They're trying to upstream it
Why would it matter if it's upstream? Upstreaming something like this is not really useful to anyone but Microsoft; it is still something that is inextricably tied to Windows and WSL. The point is not "extinguishing" Linux per se, it's achieving enough lock-in that only Linux that Microsoft customers can use is WSL.
The "default" is Ubuntu. But Debian is supported, OpenSUSE is supported, Kali is supported. Unsupported but available, you can get Alpine, CentOS, Fedora, Arch, lots of distros.