America’s interests are served in having this question not answered. If the truth is the EMP will be ineffective, we nonetheless want to bait rogue nations into attempting the EMP attack because then we’ll have the right to invade them. If the truth is the EMP will cause widespread destruction (i.e., 1 bomb wipes out America), we don’t want our enemies to know this for sure. America presumably knows the answer already.
For the opposite reason, North Korea’s interests are served in having the question answered.
So my guess is NoK is going to try to use the test as a blackmail bargaining chip.
“......we’ll have the right to invade them”.
I see your other points but we don’t have the capability to invade China or Russia or Iran. Nor would we want to.
“DPRK foreign minister alleges North Korea could do an atmospheric test soon.”
And if they do, it should be made clear to NK beforehand that the War is on.