Submarines with nukes that cannot be easily located is where the deterrence is. There is virtually no good defense against such capabilities, so this long range low altitude nuke changes nothing.
With a SLAM spewing radiation as it goes, however, you're stuffed either way.
Surely there are no private citizens buying ballistic missile defence weapons and if a nation state had enough resources to consider buying something like this, would they not also have enough resources that they wouldn't be searching for their defense armament online?
If you ride the DC Metro through the Pentagon stop, you’ll see a lot of ads for high-end weapons systems where other subway systems might have ads for lawyers and universities and such. It’s amusing and peculiar.
For the US, if a missile coming from the east is shot down over the Atlantic, radiation would probably hit Europe and Russia rather than the US.