It's a serious issue as N.K is a crazy country that a lot of people underestimate, the thing to consider is that N.K and China are really good friends, China is very powerful.
If it comes to war America will be hard pressed to find the resources to survive World War 3 (short of nuking all the opposing nations, which has its own risks and problems (i.e, nuclear winters, heavy civilian causalities, radiation etc.))
Not to mention America owes China so much money, it would cause major economic issues that America (and the world) might never recover from.

The MAD principle (Mutually Assured Destruction) should come into effect anyway, as if America and N.K (and by extension their allies) went to war and started firing nuclear weapons both countries would be destroyed.

I'm sure N.K just wanted to make people remember that they're still a problem for international relations, in my opinion it's just empty air.