You're getting two things mixed up. Homeland Security said any "terrorist attack" threats weren't credible, as in any sort of plan to attack the US movie theaters physically. Where the attack was coming from is irrelevant, they said there was no evidence of an upcoming attack. Also yes, the US is now saying that the hack did come from North Korea after all.
I never said there was a connection between credibility of the threats and the source of the hack...just that it's not a prank from a couple of teenagers (like you implied on the first page of this thread). It's a little more..."official", for lack of a better word, that's all.