I can’t see a world where RFK costs Biden a higher percentage than Trump in any state, that seems like FUD being spread by bad actors. I am not saying anyone here is a bad actor, I am just thinking that people are fearing the worst possible outcome and bad actors are praying on that to push an agenda and because of Wizard’s First Rule (“People are Stupid. People will believe anything because they want to believe it or because they are afraid it might be true.” [Amusingly that quote comes from an Author that if he were still alive would probably vote for Trump]) it spreads, just like most other misinformation.
You can’t just say he’ll cost Biden 0.1% in Michigan without also considering how much he would affect trump, if he costs Biden 0.1% and Trump 0.9% he still only gets 1% of the vote, but it hurt Trump 90% more than it hurts Biden, which in the end will actually improve Biden’s position. And the newly (within the last 4 years) never-Trumper republicans who would never-Biden as well who votes for RFK Jr can’t be counted as a “biden” vote since they would never vote for him to begin with, if anything its a vast majority of 2020 Trump votes that are shifted to RFK Jr.