You suppose there are people out there that were on the fence about voting, then heard the speaker at Trump's rally say that Puerto Rico was garbage floating in the ocean, then heard Biden say the only garbage he saw was from Trump's supporters - and those people were like "well I'm not voting for Harris now"?
Those people might well exist but I think it's fair to say that if the survival of American democracy depends on convincing people like that to not support the obviously evil candidate then we're well past the point that the derailment of the train can be avoided.
Harris gets to have her cake and eat it too, though; she can say she disagrees to give herself a little bit of distance from Joe in the eyes of the moderate right, and at the same time she gets the press bump on the moderate left from her boss saying it. Honestly, it's probably going to go well for her.