UTF-8 was invented in 1992 and was first published in 1993. Windows NT 3.1 had its first public demo in 1991, was scheduled for release in 1992 and was released in 1993.
Technically UTF-8 was invented before the first Windows NT release, but they would have had to rework a nearly finished and already delayed OS
Also keep in mind that ISO’s official answer was UTF-1 not UTF-8, and UTF-8 wasn’t formally accepted as part of the Unicode and ISO standards until 1996. And early versions of UTF-8 still allowed the full 31 bit range of the original ISO 10646 repertoire, before it was limited to the 21 bit range of UTF-16. Also, a lot of early UTF-8 implementations were actually what we now call CESU-8, or had various other infelicities (such as accepting overlong encodings, nowadays commonly disabled as a security risk). So even in 1993, I’m not sure it was yet clear that UTF-8 was going to win.
Technically UTF-8 was invented before the first Windows NT release, but they would have had to rework a nearly finished and already delayed OS