There is limited Trump-vs-Harris state polling data available. Some has to be predicted based on other polls, and there won’t be many sources to work with.
Harris polls slightly better nationwide than Biden, but it’s not by much.
However, the model they use, which makes use of information above-and-beyond just poll results, also predicts Biden to have a better chance of winning the Electoral College.
However, this model is built based on assumptions that may not hold for this particular unusual situation, and the authors are not sure how well those assumptions will hold up.