I'm neither an expert nor an american, but the idea that RFK Jr running as a third party candidate will hurt the democrats seems strange to me.
His policies, which can be summed up as "deny reality", align very closely with the modern republican party, not the democrats. It's hard to imagine that he would pull more votes away from Biden than Trump. Are there some people who would vote based on name recognition? Maybe... but surely it can't be that many? Meanwhile "Trump but not a rapist" must appeal to a number of the evangelical republicans...