I would question whether “incumbent advantage” even applies to a guy like Biden. I wouldn’t question it if we were talking Clinton 1996 or Obama 2012, but Biden isn’t the average incumbent. He’s 81, and also has very low approval ratings for an incumbent. If this were a 61-year old Biden, he might have incumbent advantage but it’s not. So right off the bat, I’d challenge Lichtman’s formula for not taking this variable into account.
Also, bear in mind that in the past half-century, incumbent presidents have lost just as many times (4) as they’ve won (4). Ford, Carter, Bush Sr. and Trump lost. Reagan, Clinton, Bush Jr. and Obama won. So a 4-4 record suggests that incumbent advantage doesn’t mean all that much anyway.