I guess the answer is the same as how the country is able to win gold medal at the Olympics  – money (and lots of it) sunk into state-sponsored training programs — but I'm a little surprised that it's effective. Even in the US, a school that would consistently produce A-grade hackers would be pretty hard to build.
I have read that this is a common tactic North Korea employs when sending people abroad for business (or, in this case, study). Their families are kept as hostages, and if any of them were trying to escape, they would all be punished. So they have to constantly check on each other for fear of their own lives and those of their families.
So North Korea doesn't need to build an elite hacker school themselves, they can just send promising candidates out of the country, make sure that they do nothing but study, and ensure their return using the threat of violence.
"In 2016 we began observing actors we believe to be North Korean" - key phrase "we believe".
"Now, we may be witnessing a second wave of this campaign"
- "may be".
The whole article is just fishing for connections with zero ties to anything solid. Even if they did connect two dots, we can now claim that a few people doing something is the entire North Korean regime? Pppplease.
anyone in the world with a proxy can pretend to be the "state sponsored hacker"
I don't see how people are still falling for this, but hey it still works and you can make millions of dollars without prosecution.