I don't believe that North Korea has so good hackers. Only the elite of North Korea has access to computers and internet. Maybe they hired Russian or Chinese hackers.
That can be a big possibility. Or they hired Chinese or Russians to teach and train a pool of North Koreans to be good at the job. With enough incentives and of course some small threats if they don't well, some genius in North Korea will have no choice but to do their best and show results otherwise their own lives can be in danger. There is nothing the government of NoKor will not do just to get the results they all wanted...including hacking banks and cryptocurrency exchanges. I am hoping that soon good countries can find ways to deal with this menace and have NoKor a taste of its own medicine.