The story here isn't that Trump doesn't understand magnets, it's that he was transparently told to say something by a donor associated with John Deere and he fucked it up because he didn't understand it.
Yeah the best part is that he jumped from elevator magnets and how they stop working when they get wet, to something about they should have stuck with John Deere. His cognitive decline is very real and accelerating quickly, and likely being fed by all his legal issues and knowing that there is a very good possibility that if he can't get himself into the white house he'll never be a free man again.
It will be interesting if they use his deteriorated mental state as a defence when he is found guilty in any of these criminal cases trying to get him a lesser sentence.