It’s BS, just like North Korea’s “nuke test,” where they blew up 200 - 400 tons of TNT or some other high explosive underground to simulate a nuke and then sprayed nuclear signature elements a week later after someone noticed the weren’t released.
Just like China shot some astronauts into space that were actually floating in a swimming pool.
It’s all saving face at any cost.
No, that is not what happened at all.
In special weapons, one of the hardest things to do is to keep all the pieces together so all the fissionable elements to so, and not just lose shape and start to be blown apart in the first few picoseconds... and as a result you lose the vast majority of your fissionable materiel.
It’s not that much of a problem with very crude devices, such as the pie and wedge Little Boy, but when you try to make the warhead smaller, and current weapon-sized, it was beyond the NORKs capability in their first test.
It was a nuclear (splitting atoms chain reaction) explosion, but a very weak “sloppy” one.