If NK have used a boosted fission weapon to join the "megaton club" then they wouldn't be the first. The UK did much the same during the early "Grapple" trials in May 1957 exploding a large boosted fission warhead (Orange Herald) to cover the fact that the true two-stage device (Short Granite) had massively underperformed. It took another 6 months or so to detonate a genuine radiation implosion two-stage weapon.
Quite ironic that, if true, Corbyn has appointed an anti-Trident Shadow Defence Secretary on the same day. Wonder if Dave was on the blower to the Supreme Leader asking if he could let off a firecracker?