The thing you have to remember about Trump is that he is incredibly stupid, he isn't that complicated. One of his buddies either made deal or wants to make a deal there the idea of "Denmarkians" full of whites joining the US stuck in his head. He couldn't find the country on a map let alone know the population make up, land mass or any of that, in his head he probably sees it as one giant real estate deal and a country he can rename after himself.