I think the original notepad.exe was just a Win32 Edit control (whatever it was called) with a window and some menus. I expect that Apple's TextEdit.app is just a wrapper around the rich text control in Cocoa, too.

But yes, it's hardly writing a text editor to write a Win32 app in assembly. (Although, if they used the COM control and did that in hand-written assembly, that would at least be an impressively tedious mortification of the flesh.)