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The Windows Subsystem for Linux is now open source.
(blogs.windows.com)
This is a most excellent place for technology news and articles.
You are confusing the software and Linux kernel that make up WSL2 because the former was proprietary until now and the latter has always been available on GitHub.
Kernel used in WSL2
https://github.com/microsoft/WSL2-Linux-Kernel
That's news to me. Thanks for the info