Japan is further away from breakthrough capability in having a viable nuclear weapon than they were, say, like two decades ago. But if they funded a crash program they could certainly do it. South Korea is another country which could have nuclear weapons if they wanted to.But that guy is dead so his prediction is free to be changed at any time depending on if the Japanese government and the US government is willing to make this arrangement happen because with an economy like Japans right now, it doesn’t have the investment capabilities to make this happen not to mention that the USA no matter what they say is never going to let this happen
The key is to look for any country going for these technologies: uranium enrichment, plutonium separation, multiple stage solid rocket technology. South Korea and Japan had AVLIS uranium enrichment programs at one point, Japan used to have facilities to handle MOX fuel. And Japan has a multiple stage solid rocket 'Epsilon'.