Criddell is right about the history, and Xenix was literally not Unix either. At some point Microsoft agreed to not compete in the "Unix" operating system market, broadly defined.

An awful lot has changed since then, including Microsoft buying a System V Unix license when SCO was extorting everyone (including Sun, as I recall) into doing that. There is so much water under the bridge, I imagine Microsoft could do it if they wanted to. But why?

I don't see how it's a good idea from their point of view. An ubuntu respin is about as far as I'd ever expect them to go down that path, and even that seems like more commitment than they'd want to make.