The North Korean hackers are not just targeting the South Korean Bitcoin owners, they target every country in the world. <The easiest target are individuals with little knowledge about security and also smaller exchanges with weak security>
Also remember that North Korea has some allies and they circumvent these sanctions via their allies, so they are not just using technologies like Bitcoin to circumvent sanctions from their enemies.
They are always one step ahead, when it comes to things like this.
North Korean is the empire of smart people. Large countries influenced the North Korean by sanctions but the country still developed.
North Korean has face-to-face allies, and they support the country through various ways to eliminate the effects of sanctions from big countries.