It's just the Art of War. If you don't have a strong program yet, say nothing about it and let your enemies wonder about your capabilities. Just because they may have one undeclared test doesn't necessarily mean they could succeed with a public MAD strategy; they may need many more nukes than they currently have.

If they say they have none, their enemies might think there's a chance they actually have a lot, or at least enough to cause serious damage. And there's the deterrence. If they say they have a lot but don't demonstrate it, their enemies will probably think they're bluffing and will demand proof. I think it only makes sense to go public when it's a gun that can be pointed directly at someone's head, rather than a few of their toes. It's probably the best way to project a strong posture despite not actually having one.

And more practically, it lets them develop the program with less chance of other countries (including allies) complaining about it or condemning it, which could potentially slow it down or stop it before it's sufficiently developed.