IANAL, but I'm skeptical that Dmitry's interpretation that Defcon has no license is correct. It sounds like Dmitry sent them firmware images with the mutual expectation that those will be used on badges, and they invited him to the Badge talk which could be considered consideration. That should constitute a contract, either verbal, or through concludent acts. This should give Defcon the right to use Dmitry's on the badges, but not modify it. So legally the whole thing would probably be considered a contract dispute, not use of unlicensed software.

Defcon will probably argue that including the easter egg was some kind breach of duty of Dmitry's part, and gave them the right to remove him from the talk, and modify the firmware to remove the easter egg. My expectation is that courts would decide that Defcon has the right to use the firmware, but will require them to pay some kind of compensation for not living up to their side of the bargain.