Digital signature in network protocol

It may be a bit offtopic, but I'd like to know what expert ppl on this board think about this -
At hxxp://lkml.org/archive/2003/7/17/124/index.html you can see the message from the owner of the BitKeeper software, proprietry source code management software. According to that message, he says he can "put digital signatures into the protocol to prevent your clone from interoperating with BK." Is this possible at all? When I can crack and made up exactly the same signature from cracked/clone copy to valid copy to emulate it, such digital signature seems to mean nothing. Am I wrong somewhere?