It hinges on the definition of "derived work" (anything outside derived work is outside copyright's effects, and the GPLv2 uses only copyright to give it effect

It hinges on the definition of "derived work" (anything outside derived work is outside copyright's effects, and the GPLv2 uses only copyright to give it effect). Their WSL2 Linux kernel will clearly be a derived work of the Linux kernel, so they obviously will provide full source code of that; the rest of Windows most probably won't.