Not "define" as in changing some words. If they actually made an OS that was almost entirely IE, then sure, it would be pointless to ask for other browsers to run on it - because running another browser would mean replacing the OS with something else, just like with Firefox OS.

The only OSes like that are Chrome OS and Firefox OS (edit: and WebOS). Windows, Android, iOS, etc. are not browser based, so it make sense to ask for other browsers to run on them.