The subsystem that was made just to claim POSIX compatibility was the subsystem that preceded Interix. Interix was developed outwith Microsoft, and replaced the old POSIX subsystem. It was Windows NT's second POSIX subsystem, and it very much is not just a sop for the sake of checking a requirements checkbox.
It's a fairly usable POSIX environment, with a large toolset, proper terminal emulations, pseudo-terminals, interoperability with non-POSIX processes including visibility with (an enhanced) ps and the ability to send signals to Win32 processes, an actual init and inetd, and an X server.
Stephen Walli has stated that it started as a fork of 4.4BSD-Lite.
