Who knows if what KJU was viewing was an actual hydrogen warhead, or a dummy for propaganda purposes. NK’s first missiles were ludicrous failures.
I would consider it quite odd that we hadn’t seen any tests first from rogue H-bomb builders. We’ve seen NK test fission bombs, but not a fusion bomb.
Lots, and lots of people were surprised with how quickly North Korea has been advancing, with the very difficult technology of intercontinental missiles.
Don’t assume then cannot make stuff.
Quickly.