I think it is a distraction. I do believe they are at least somewhat capable of the threats they speak, but I also think it's because they have support in the background and they are being used by the same.

Who the powers that be in the background are I have no idea, few guesses would be China, Russia & Iran.

China is escalating towards Japan
Russia has be giving a show of strength with fly-overs
Iran and the rest of the middle east & north Africa are in turmoil right now as well.

All three above have their agendas and want to keep the US and her allies out of their business so they whisper in Kim's ear and make promises to him that they will back him up. He keeps us focused on NK, meanwhile the other three are able to further their agenda.

This also gets many of our troops off US soil and spread out all over the world.