I disagree with the idea that the DPRK is not capable of such attacks. They've demonstrated they are capable of creating nuclear weapons and ICBMs, I don't think it's such a stretch to say they are capable of APT attacks. Especially when you consider that the DPRK gets a lot of support from China, which also engages in cyber attacks. It wouldn't be difficult to send DPRK citizens to China to be educated and instructed in computer technology. More difficult, but still possible, is covertly sending north koreans to various western institutions to receive an education in cyber security. Recall that Kim Jung Un himself was educated in Switzerland.

There's also the possibility that the APT group is partially or completely composed of foreigners hired to engage in these attacks on behalf of the DPRK. I know one of the people on Operation Blockbuster believed that this was "contracted out" to some clandestine group. This would explain the attackers being multi-lingual. It would also explain why many of their attacks focus on financial institutions and theft. This focus has been proposed as a way for the DPRK to get international currency to make purchases and evade sanctions, but it would also make sense for a group that is in it for the money.