You’re right. By the peace treaty after WWII, Japan has agreed to limit their armed forces to purely defensive and build no aggressive capabilities. Certainly no nuclear weapons. In exchange, we guarantee their security.
It has been an open secret in DC for years that Japan has nuclear weapons. Probably about a dozen or two.
The took all their breeder reactors offline in 2016, but easily had plenty of Pu by then.
Fissionable weapon design is so well known that there is really no need to test these days.
The only question is, what will we do when Detroit gets The Bomb?