The first is that he is just experiencing the ravages of age. Trump is exceptionally old for a Presidential candidate and they say the memory is the first thing to go. It’s entirely possible that Trump literally does not remember that he said that Japan should get nukes. I don’t know why someone doesn’t remind him that he said it before he goes out and claims that he never said it, but
Trump isn’t surrounded by competent people, so that’s pretty much all the explanation you need.
The third is that he assumes that no one will care, and that this assumption is based on how his followers to this point have behaved. Certainly, the 43% of Republican voters who voted for him in the primary have not shown any sort of willingness to be swayed with facts, or to care about the fact that Trump is a pathological liar, but Trump’s assumption that this will translate to the general election is a bold one, indeed. (Continues with video included)