However, there is an age old equation that works fairly well for attribution: opportunity + motive + evidence. Opportunity in cyberwarfare is a complex item to tease out and has more to do with opportunity costs than anything else (the strategy of cyberwarfare demands owning everything immediately rather than later and the exercise becomes one of triage).
In this instance we do have both a motive and some evidence that point to North Korea, and it is in line with the sort of theatrical politics NK is known for. IMHO more evidence and an analysis of opportunity cost are a must to pin NK down further, but you can be assured that (unless officials have ulterior geopolitical motives) they have a pretty good idea about the cyber capabilities and actions of other nation states - more than they are apt to release to media outlets.