From the article, it wasn't a XOR, the "key" was that there was a bit sequence when interpreted as Morse code said something like "copyright ATARI". So, reasonably easy to show it wasn't just pulling the message out of thin air.

The ROMs wouldn't an exact match so you have to convince the layman jury that the differences are meaningful. The best way it to show something totally unexpected in the pirated code like your own copyright.