You either believe it or you don't. There have been instances where more details were released (the Clinton campaign hack, IIRC), and it just gave more opportunity for people who did not want to believe it to nitpick details.

Whoever does such things doesn't exactly sign their exploits to prove ownership. So the evidence might be something like IP addresses in log files: totally convincing if it's your log file, but so easy to manipulate it's useful to convince someone who does not trust you.