Yep, this could very well be the Mother Of All Bluffs. As to why Kimmy would do it, I suspect he's looking to sufficiently frighten the West (and the U.S. in particular) into not making any military moves against his regime, thinking we'd be unlikely to do so if we thought he had nukes.
I dont think that they were testing a large device, I think that they were testing a very small device, suitcase device if you will, that can be transported either by hand or in a small vehicle. Ponder that for a bit.