This is possible because Dave Cutler and the rest of his VMS team built Windows NT to support multiple personalities. From day one the Win32 subsystem was just

This is possible because Dave Cutler and the rest of his VMS team built Windows NT to support multiple personalities. From day one the Win32 subsystem was just one of several. That idea got sidelined when Windows became so popular but the core has always been there. It's kinda funny: the Windows NT kernel (and Executive, the actual native API) are a single-root object hierarchy where everything gets mounted - much like unix - only even moreso. Mutexes, Registry Keys, etc are all objects mounted in the root filesystem. I guess everything old is new again. Microsoft used to have a server product called Services for Unix that replaced the built-in POSIX 1.0 subsystem with a legit Unix environment but it was always a neglected product that only existed to get people to port server apps to Windows. This time they're doing the integration on end-user desktops and have spent more time getting Win API integration working smoothly.