The 400kt is wrong. That's not a number one could even imagine getting with the first nuke test in one's program. Either the administration official made a mistake to FOX or it's pro-US spin.
Also, military officials wouldn't even be saying that it was a nuke test if they didn't know that it was a nuke test. Again, the seismograph is very characteristic. The USGS has record of a 3.5 from NK. (Not up on the website yet, and according to some here, it may never be.) I'm sure our officials have already analyzed that, as well as other information, to come to the conclusion that NK did conduct a legitimate and successful nuke test.
I should add a third option: Total overexaggeration from NK of what they expected for bragging purposes. However, the NK scientists surely did not expect 400kt, I can guarantee you that.