What are the chances that some of North Korean nuclear assets do not actually exist ? They might have some real assets the rest might be imaginary. After the Cold War it turned out that Soviet military might was lesser than estimated. The same was with Saddam Hussein's regime. Imaginary assets might serve two purposes. First, the deterrence. Second, for "domestic consumption". Think of Potemkin Villages. Someone must have taken credit for creating those assets and benefited from it. Openly admitting that assets never existed migh get someone in trouble. Totalitarian regimes can control the flow of information in and out, that gives them an opportunity for deception. My guess is that intel on NK nuclear assets is, at least, partly obtained indirectly. The sources might be in Iran and Pakistan, the countries where NK nuclear program got assistance from. Those sources can be manipulated on purpose.