The US's massively expanded nuclear capabilities would rightly be perceived as North Korea as a threat, partially because when the military talks about using those bombs, they often pick North Korea as an example target. So they've adopted a MAD strategy insofar as they're able.

The US's nuclear sabre-rattling only tends to be reported in military-centric press, but I'm guessing North Korea has a google alert about themselves and have seen the threats that most USians haven't.