"Nuclear attack capacity" has an exact meaning.
It's a long way from lowering a truck-sized "device" into a mine shaft for a test from getting one that fits on a missile nose cone, or will detonate under less-than-optimum conditions.
Given the typical dictator's tendency to over-exaggerate one's own weapons capacity (remember Saddam?), I'll believe North Korea has any nukes at all when they set one off and world observers confirm it. It would have to be more than seismics, too.