It's not that they wouldn't be able to. It's that proving it was them is damn near impossible. The claim that it's DEFINITELY THEM is just so hard to put any real substance behind that it just appears to be fictitious. The amount of proof that you could find in a single binary is very limited, and even that stuff could be faked on purpose. To say that it must, 100%, without a doubt, be of North Korean origin is about as foolproof as any other conspiracy theory.