It is my understanding that plenty of people do not believe North Korea cheated in 2010 (and also, plenty of people believe that other specific teams have cheated in specific years). Furthermore, it is my understanding that the North Korean team leader in recent years has not seen the problems before the contest (relying instead on the South Korean translation of the problems), thus removing the most obvious mechanism of cheating. Unfortunately, I do not know of a public source of this information and accordingly I have not verified it myself.
And yes, somebody who has the skill to craft a very low level yet highly flexible malware requires skill that goes beyong copy/paste from stackoverflow.
I know enough math people who are ABYSMAL at coding due to the differences between domains and the constraints that shaped their way of thinking, and a lot of linguists who are surprisingly good at it due to just that.
I dismissed programming as a freshman because I was so bad at math in high school. Turns out my high school teachers were very bad teachers (shocking when they're paid so little, who would've thought? ) I learned Italian fluently, spent a few years overseas immersing myself in it, came home and had a rough time finding my passion. Took a gap year and messed about with some online coding classes (code academy) and realized I loved it. It felt like a new language that instead of being expressively emotional it was a way to express logic and rules. I like logic and rules.
So I completed the minor +4 higher level classes and taught myself mobile development. No regrets... Besides not going into it as a major maybe.
Developing cyberwarfare capabilities seems like a relatively low cost (compared to developing ballistic missiles at least), low risk investment, with a potentially large payoff, so I would be surprised if they didn't try to develop these skills.
Of course, all evidence tying N.Korea to this specific attack is circumstantial and almost impossible to definitively prove unless they admit it and show proof themselves.