Well, remember the Ryongchon explosion happened 2 years before the first nuke test in 2006 and it was a huge amount of ammonium-nitrate fertilizer? Why not use the free fertilizer that you get from Japan and South Korea, turn it into a bomb, blow up a tunnel, and terrorize the world with it?
A nuke has a much faster rise time than any chemical explosive. Makes a different signature.
We’ll fly a spy plane with an air sniffer to look for fugitive isotopes to confirm.
We have shot many “huge piles of high explosives” and know what they look like seismically, like Misty Picture and Minor Scale.
With that kind of a track record, I’m sure the South and ourselves kept track of what went in before the explosion.