People make fun of him, and I'm not going to go into whether it was responsible to phrase it the way he did or not, but in my mind the most likely explanation a

People make fun of him, and I'm not going to go into whether it was responsible to phrase it the way he did or not, but in my mind the most likely explanation as to why he said it was that some scientists explained it to him and he just regurgitated what he heard without putting it into context (I don't imagine Trump is particularly technically inclined).