Biden chose his words poorly. What he got most wrong is that he assumed a context he didn’t bother to explain, which meant he missed an opportunity to educate anyone interested in learning.
Biden should have responded by proclaiming that he will never raise questions of fraud or illegitimacy without hard evidence. He could have reminded everyone that he had attended the inauguration of quite a few Republican presidents, including two who won very close elections, and accepted the results every time. He could have said that he hoped that every election is free and fair, and that he’ll do everything possible to see that they are.
Biden also could remind people that he’s old enough to remember the days before the 1965 Voting Rights Act, when elections in many states were not free or fair or legitimate; that it was one of the great achievements of both parties to correct that injustice; that he’s especially proud of his Democratic Party, which was once the main source of undermining electoral democracy but had done the hard work of reforming people or removing them if they would not reform.