Realistically, there is no way NT could have used UTF-8 in-memory. UTF-8 was scribbled on a placemat on September 2, 1992 and officially presented in January 1993 (https://en.m.wikipedia.org/wiki/UTF-8#History)

Windows NT was released in July 1993, but development started in 1989 (https://en.m.wikipedia.org/wiki/Windows_NT#Development)

Also, even after UTF-8 was accepted to be a good idea, it wasn't considered a good idea for in-memory usage; indexing UCS-2 encoded strings is way easier, and UTF-16 didn't exist yet. It arrived with Unicde 2 in July 1996 (https://en.m.wikipedia.org/wiki/UTF-16#History)