What difference does it make what RFK believed unless it was based upon some evidence that the rest of us are unaware of? That's even assuming this is an accurate characterization of his viewpoint for which there is cause to doubt. It's pretty damning of RFK's reputation to suggest that he kept silent about unknown conspirators getting away with murdering his brother. He was certainly in a position to raise awareness of his position if that is what he thought happened.
If RFK publicly criticized the Warren Report, it would've pretty much ended his political career.
It made no sense for him to do that unless he had a smoking gun or hard evidence of a conspiracy.
My broader point is that its telling that so many people who were close to the Warren Commission or knowledgeable of the intimate details were also not convinced that they were right that Oswald acted alone. That includes Lyndon Johnson, Hale Boggs, Richard Russell, RFK, John Connally, and possibly Gerald Ford (if some quotes attributed to him are true).