Yeah, I'm very skeptical about the claims that north koreans are doing this. It just doesn't seem possible that they could have built up such an advanced level of exploitation when they have such minimal infrastructure.
Building up an offensive computer hacking operation is relatively cost effective compared to building a military. All that is required is a few computers and some expertise. Cyber warfare is perhaps the only battlefield where North Korea could compete with world powers. With these crypto hackings, the operation probably pays for itself many times over