A large percentage of Taylor Swift’s most devoted followers consists of teenage girls who can’t vote. Much of what remains after that cohort would never vote for Trump under any circumstances. It is reasonable to conclude, therefore, that a Biden endorsement from Swift would make little practical difference in the election.
But Trump was a celebrity long before he was a politician. He still thinks like one. One of his favorite insults revolves around the television ratings of his opponents. As a result, it makes perfect sense for him to feel threatened by someone who is an even bigger celebrity than he is.