I find it amazing that you can have such a functional Ubuntu environment by translating system calls. Microsoft does have the advantage of Linux being open-source I suppose, while the Wine project had to reverse engineer DLLs. Or have you supply them on your own.
Its ironic (old)microsoft exerted so much effort to put the personalities in place (OS/2, posix etc), then (mid)microsoft systematically destroyed that work under Balmer, and now (new)microsoft are reimplementing the same thing under a (seemingly) completely different system.
I was interested in that too, since I write Unix utilities and it is sometimes useful to have them work on Windows as well. I've done that (checked that it worked on Windows) with a few utilities in the past (written in C), that did not use any very Unix specific features that were not present on Windows. And remember reading around that time that Windows (I think it was from NT onwards) had a POSIX subsystem.
Then more recently, as in, a few months ago, I wanted to check that out again, and did. IIRC I read (maybe on a Wikipedia page) that the POSIX subsystem is not present in Windows any more.
"Windows NT was designed from the start to have modular subsystems. It was most infamously used to provide a POSIX subsystem which really only checked boxes on government acquisition forms. :-)"