The key wasn't used in a book or in the hex values for a flag. That's like saying the formula for Coke can't be proprietary because it could be put in a book.
Software can absolutely be proprietary, and that key is part of the software.
I disagree. Sure, companies have a moral right to recoup their R&D costs on a console, but I fully reject the Divine Right of Shareholders. As long as the emulators aren't sold for profit and no one is hurt, a multibillion dollar company like Nintendo has zero moral ground to tell us that we cannot emulate consoles that we have bought to play games that we also bought.