Exactly, but for a few decades now the US has been trying to do so.
For the record, it's also not clear with Status-6 is operational, either. But the technology behind Status-6 is pretty much all stuff that we already know they have working, whereas HGVs maybe not so much.
US Generals have said quite plainly that Russia and China are simply ahead in hypersonics, and I'm sure if the demonstration of Avangard to the US was dodgy they'd be called out.
Also, it's not about rationality at all, North Korea has so far behaved perfectly rationally.
Multiple contracts were awarded for space-based missile tracking system that were specifically meant to deal with HGVs, so yes, there is another missile defense in the pipeline, that Pompeo literally called "SDI II".