That's way way later. UNIXes were the ones running in 70s and 80s. WindowsNT and Linux only later came to take a slice and, still later on, Linux won the day. How anyone would consider UNIX "niche" is beyond me.
DEC's VAX/VMS became Open VMS when UNIX/POSIX support was added in the early 1990's as well.
Other mainframe systems added similar POSIX environments.
The way Windows NT OS linage works isn't something novel, it is how mainframes have introduced multiple OS personalities and virtualisation modes since the late 70's.