The parser and renderer is a pre-existing RichTextEdit widget. Add menus and save, open, and print functionality and you are over 90% of the way to a WordPad clone.
https://github.com/microsoft/VCSamples/tree/master/VC2010Sam...
I'm assuming that's not built into the widget or into Windows. Is there an open-source drop-in docx<->rtf library out there?
Since there was now a RichText control that supported RTF in Windows, the Windows group could write a thin wrapper around the RichText control and replace Microsoft Write with Microsoft WordPad.
Support for Microsoft Word file formats came later once the Word group took over responsibility for the RichText control code.