Instead of comparing it to Windows, which probably has a license the lets you make software for it. How about comparing it to a console. If you figured out how to make a game for the Playstation without using their SDK, then made a great game and sold it. Could Sony win the case against you?
That said. It's probably best to check with id beforehand. You might win if the case in court, but can your really afford a legal battle with id or any big company for that matter?
That said. It's probably best to check with id beforehand. You might win if the case in court, but can your really afford a legal battle with id or any big company for that matter?