I still don't understand what it means to claim that a president is "illegitimate."
Without defining the word before asking it, there is no way to know what that poll result truly means, but I'd venture a guess that most people think that Trump didn't "win" fairly (or in other words, he cheated, whether it was literally changing votes or just getting Russians to hack his political opponents and release embarrassing information)
Those are reasonable guesses, but like the poll that showed most Dems favoring impeachment and Republicans embracing birtherism, I suspect lots of people are using this as a shorthand for disapproval.
