Earlier quoted context omitted.
Do they have the wherewithal to pull off a deliverable Teller-Ulam thermonuclear warhead, without doing any atmospheric testing? Does Japan even have SLBMs or bombers? It takes a lot more to be a credible nuclear threat than just having some U235 sitting around...
I can't speak to the delivery aspect, but I don't think you'd even need a Teller Ulam thermonuclear warhead to make a credible threat to a neighboring power. But the delivery aspect matters a lot, obviously.
Granted, I haven't read a ton of analysis on this, but my guess would be that you do if you're going up against someone with their own thermonuclear bombs. If China drops some of their own thermonuclear weapons on Tokyo, what's Japan's response? Dropping a fission weapon? If they got to that level, I'm sure that would be considered an acceptable risk for China. The converse is also true. If Japan dropped the fission weapon first, they'd instantly have a bunch of thermonuclear warheads headed their way.