Who really thinks they could mount one of these on an ICBM and hit a US target? I guess the real issue is, the guy is definitely crazy enough to proceed with the test when they don't know what their doing.
Do I think he could do that? No, I seriously doubt that he could do that.
Do I think that they could put one in a shipping container and get it into the US some how and get that into a city center? There's my concern.
Whatever device they could construct could be mounted in the hold of a ship and thence launched on an aqueous trajectory of any desired distance.
That should be a concern too: that they could actually explode a nuke somewhere friendly, but by accident. They really make the Iranians appear somwhat rational :)