That’s not how IP works.
Blizzard didn’t forfeit their rights to the IP at any point. Even selling them a grab bag of stuff that unintentionally included a copy of the source code doesn’t mean the recipient actually received a legal license to the IP.
You can make all the arguments you want, but in the court of law you’re not going to get away with anything that involves giving away another company’s IP, even if they accidentally let you see a copy of it. “Finders keepers” doesn’t work with IP.