With all of the sanctions and shit NK has suffered if they possessed this kind of technical prowess or knew parties that did they would have been hacking shit in the US a long time ago. This attack on Sony was like most hacks, a crime of opportunity. Sony is of no value to hackers, and NK has much bigger grudges against much bigger enemies than this. I wouldnt be surprised if this group recognized the trouble they were about to get in and decided NK was more or less a perfect patsy to let it fall on since people started making such assumptions.