“Not all Trump voters are registered as Republicans.”
Exactly! And, the percentage of independents who are highly motivated to vote for Trump vs Biden are vastly greater than 50%.
Reading through this I saw that the writer is assuming that about 95% of voters in both parties will vote for the party’s candidate, and that a slight majority of Independents will vote for Biden. I don’t see anything from Biden that would inspire a last minute break for him from Independents, and I don’t believe that 95% of registered Democrats are voting for Biden or that 5% of Republicans are voting for Biden.