Let's call this a plot idea for a spy novel.
There is none/zero/no proof for any of this:
The fundamental question - where did they get the technology and materiel to build their missiles and their bombs?
Where did they get the fissile material?
How did they learn so fast how to miniaturize the atom bomb and make it a hydrogen bomb?
Could it be that, after agreeing to the accord worked out by John Kerry in his final days in office, the Iranians moved everything out of their mountains into NK's mountains? Could it be the two countries agreed to share the program, with the NKs fronting the operation while the Iranians provide scientists, supplies, and money?
I can imagine stranger things.
And as I said, there is, to my knowledge, zero hint that this is what's happening. Just some fun thoughts.