It's not just that he will take more votes from Trump than from Biden (or Newsom), it's not even that the Dems will make up any lost votes in the cheating while using his candidacy as cover for extra cheating (who would look for votes switched from Trump to RFK?), it's that he is objectively worse than Trump even if he somehow did win.
If he runs 3rd party it will have been his plan all along to hand the election to Biden for his Dem friends.
This seems most likeley to me