I think the 'specifics' of Fredo's betrayal are made perfectly clear in that classic boathouse encounter. Fredo expresses rage and jealousy at his being passed over in favor of his younger brother. Probably for the first time. Even without the deleted scene between himself & Deanna at the film's opening (..."Because he's head of the family."...), we are made aware that this has been eating away at Fredo for years. He went after the chance at something for him, on his own; and even though he wasn't aware of a planned hit...was perfectly willing to sell out his brother in order to get it.

I don't think FFC left us in the dark at all about Fredo or his betrayal.

Apple