WindowsNT was the other operating system of that time.

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.

In the other direction then, IBM mainframes.

With Win32, OS/2 and POSIX subsystems.