The U.S. authorities claim North Korea is behind it. They are not "amateurs".
I would really like to see a detailed explanation of the authorities' evidence that North Korea is behind the attacks, but of course we don't get to know what evidence they have, we just have to trust them.
Until I see something plausible, I'm just gonna assume this is another US:NK playbook exercise, designed to make NK seem like a real threat to the West, and to give an excuse for more defense corporations to sell contracts to Western armies.