Just because it's "strictly business" doesn't mean that North Korea wasn't involved. They probably know how to short stocks too.
The broken english used in the threats is a match to a google translation from gramatically correct Russian. That doesn't seem like a coincidence to me. Since the Russians hacked the NASDAQ as recently as July 2014, maybe they had something to do with it. And Russians are known to enjoy manipulating stocks
Mind you, I don't think this has anything to do with manuipulating stocks. I think it is far more likely that it was some person who didn't like Sony very much and the deflection onto the DPRK was just a red herring. But if shorting stock WAS the angle, the Russians have a lot of experience doing it.