They’re making this argument about Ukraine, and yet are perfectly fine with the US on their doorstep.

... what? The DPRK regularly calls for death to the genocidal Imperialist US Empire. What are you talking about?

It's been the official policy of the DPRK for most of its existence that the US was its primary enemy. That recently changed to South Korea, but the US is still enemy #2 (I disagree with the new characterization, but that's neither here nor there). The whole reason the DPRK is so aggressive about its own ICBM program is to deter the US. It can attack Seoul just fine with artillery (which is part of why it has maintained such an extent of artillery manufacturing that it's exporting to Russia).