Except there's no way of telling whether those addresses weren't being used proxies too.
This is an exercise in Bayesian logic. If you had a high degree of prior suspicion that NK was behind this, it'll look like a smoking gun. If you have a low degree of prior suspicion, it won't look nearly so significant. Personally, I'm in the middle. I think this makes it more likely that NK was behind the attack, but I don't regard it as a "smoking gun". It seems perfectly credible that someone who can orchestrate the Sony hack could hack an NK host. We know that the attackers *sometimes* used proxies. So which is more likely, that the NK addresses are just another red herring, or that they "got sloppy"?
The reason for my agnosticism is the sheer diversity and chaos of the Internet. Arguments that "it makes sense" for so-and-so to have done something hold no water with me, because there are people out there who will do things for reasons that make no sense to me, or won't do things when I think they should. It makes perfect sense for NK (as we understand them) to be behind this, but that doesn't signify.
Motivations are weak evidence for anything. It's like me and my brother-in-law, who is a big-shot cultural studies professor at a prestigious university. I once mentioned to him I always wanted to have a Unimat -- a miniature desktop machine shop. This totally mystified him. He couldn't imagine why someone would want to have such a thing. On the other hand, if I'd said I'd wanted to meet third wave feminist philosopher Judith Butler he'd have found this perfectly understandable and logical. Many people who understand the attraction of mini-machine tools might not understand the appeal of meeting with a major post-structuralist thinker, and vice versa. Unless you see the attraction of both, your understanding of one or the other group's motivations is bound to be unreliable.
Our reading of other people's motivations is apt to say more about ourselves than about them. Hard evidence is what is needed before motivations can contribute to our beliefs one way or the other. Tracing the attack (in part) is a step in the right direction, but far from conclusive.