MS tends to keep a lot of fingers in a lot of cakes, even in surprising areas. Given the number of kernels MS Research has released, it wouldn't be too terrible of a stretch to see MS release some sort of unix at some point.
I think the big thing that would keep MSFT from rolling their own is fear of antitrust law, in the US and Europe. Less of a concern for them when it comes to Linux on phones or embedded, I believe.
From that article:
"according to AT&T, roughly half of the 500,000 Unix licenses are held by Xenix developers"
I really am amazed by that figure, though. Wow. And I had forgotten that xenix ran on some bigger machines, not just pcs.
They later bought Interix, who created the foundations of Windows Services For Unix, which was a much better and actually usable Unixlike layer, running directly as an NT subsystem, at the same level as Win32.
(This is as distinct from Cygwin, which supplies a GNU layer on top of Win32.)