RFK Jr, like the other third party candidates, is running as a spoiler to help Trump — his staff have even said so:
The only way that Trump can even, remote possibility of taking New York is if Bobby is on the ballot. If it’s Trump vs. Biden, Biden wins. Biden wins six days, seven days a week. With Bobby in the mix, anything can happen
He’s not even running as a Democrat. Once it became clear that Biden was going to get 80%+ of the vote in the Democratic primary, RFK Jr decided to run as an independent, but to try and trade on his name, rather than run on the pro-measles positions he’s been pushing for the past couple decades.