Well, Xerox saw no real use in the mouse or the GUI, and so didn't bother to patent them (there were a few others, but these were obviously the biggies. My mother worked for Xerox back in the day.)
"I'm confused. Are you classifying the underlying code that makes up the OS as a language?"
I'm sure he's talking about the code. There were actually quite a few versions of DOS, such as Dr. Dos and such.