Show me one example where he's said something like that and he was citing a real source. And I mean an actual source that he should be respecting. If he just parrots this stuff from other cranks on Twitter, that's not exactly the defense you think it is.
This specific issue is based on a lawsuit, googling brings this up:
https://www.eastcountymagazine.org/robert-kennedy-jrs-legal-...
I can't speak to whether the case is good or not, but the point stands: these aren't his original claims as so many people like to say. They come from e.g. expert witnesses. Maybe those people are cranks, I have no idea.