DPRK is able to say the things they say because they have an ace up their sleeve. They do not have any better weapons than anyone else, but may have weapons that are "worse" in relating to things even the US had made in the past but would not use. But the real threat is not from them. The circumstance being reported in the news is not an official one, it is from an editorial, but that won't stop the MSM. Those circumstances are:
1. If DPRK is attacked first, China was defend.
2. If DPRK attacks first, China will not interfere.
However it is more than that, due to the history. Even if DPRK attacks first, you can be sure that once foreign troops step into DPRK, then China will help to defend. North Korea is now less actually Korean and more Chinese. They would not exist if not for China.
So when Kim Jong Un comes out and says anyone who attacks them will suffer their doom, they are not saying that North korea will be the ones who will be rending that punishment, they know that China will have their back.