It is more likely that an elite group in Pyongyang has internet and computer access, with a subset of that group given more privileges to engineer & deploy malware/etc.

Given that though, I find it rather hard to believe the North Korean government could pull off a cyber-attack like this all by themselves, especially when you take into account the effect of international sanctions and the North's very outdated technology (I remember reading they still use a 1960's crank-phone for communication through the DMZ).

My best bet is that the Chinese government gave them significant assistance in launching this attack.