I could see that.. An actual hydrogen bomb requires significant quantities of tritium and deuterium. You get those via centrifuging ocean water then cracking the hydrogen. (Or, you can enrich the hydrogen by exposing said water to neutron emissions of the correct hardness for extended periods, THEN centrifuge and crack.)
A heavy water processing facility would draw the immediate attention of the various international nuclear regulatory agencies, because there really isn't a whole lot you do with heavy water on that kind of scale-- at least nothing NORTH Korea has demonstrated serious interest in. (It is used in a number of theoretical physics experiments, some medical applications, and a few others. All are things that Best Korea has shown almost pathological disdain for.)
The lack of appropriate processing facilities, the incorrectness of the seismic data of the device test, and the bombastic nature of Best Korea all kind of say that this was not a genuine staged nuclear device with a powerful secondary fusion cycle based explosive-- EG, an "H bomb". Rather, it looks like either a conventional fission only nuke, like they have done in the past, or a boosted nuke, like Sheb suggests. (that would require significantly less pure, and less total quantities of those isotopes of hydrogen.)
Really, if Best Korea was trying to get attention, they could have done it with far more seriousness if they had stated that they were going to build a heavy water processing plant to further their nuclear program, then proceeded to do THAT instead of poisoning the ground someplace in their tiny little backward plot of ground for centuries to come. But hey, PR machines are rarely known for being sensible.