I was suspicious of the U.S. allegations that the North Korean government was behind it when the North Koreans denied it was them. If you're going to hack somebody to make a political statement, it makes no sense to later deny that you were involved. Someone might be trying to make it look like North Korea, but I seriously doubt they were directly involved in this.
Wrong--Even implausible denials can be very useful in international relations. They give sympathetic expatriates and foreigners something to support and are also useful legally. The obvious example is Putin's recent doublespeak over invading Ukraine. It is only a paper shield but it helps confuse the issues slightly, delaying and discouraging organized response of any kind.
As another example, since the UN Charter as passed, open wars of aggression have been outlawed. As a result, there have been a whole lotta agressive "self-defense."
As another example, Israel-Palestine. Regardless of which side you're on, you'll see the other side doing what you think is lying about something or the other.