The Office apps have used their own controls for a long time. Check out Word 97 running on Windows NT 3.51 for example: http://toastytech.com/guis/nt351word.png / http://toastytech.com/guis/nt3513.html
In the past they would either land in Win32 classes or updated MFC.
The ribbon introduced by Office eventually became available as a COM UI library.