He didn't say they know they won't win, he said they knows they aren't changing anyone's mind. I think that's true.
I think that is generally true with incumbents, but particularly in this case, Ican't believe anyone who hasn't already made up their mind on him by this point is going to be swayed in either direction by anything he does over the next 7 weeks.
It's basically whether the Democratic base is energized enough to vote, and whether Biden can make fiscal conservatives who dislike Trump comfortable enough to vote for him