If N. Korea makes one concession or move, it has the next step, especially the "worst case" next step mapped out. So if it admitted that it has been working on nukes, it has enough of finished nukes to give their adversary some heartaches. But N. Korea will go there in slow steps, trying to incur maximum psychological impact in the process.
They are taking NPT regime as their hostage. If N. Korea publicly deploy nuke weapons, that will be the end of NPT regime. After that, there will be all-out confrontation. So N. Korea will be using this doom's day scenario as their bargaining chip.
I think that this is why N. Korea is suddenly going in peace offensive to S. Korea and Japan. They want to contrast America's approach and the others, the clear contrast of all-out war and the peace on N. Korea's terms. They are trying to isolate America. That is their intent. It is another question if the event will play out as N. Korea wishes.