I'd like to come at this from a slightly different angle:

In the boathouse, Frredo told Michael: "Johnny Ola bumped into me in Beverly Hills -- and he said that he wanted to talk -- he said that you and -- and -- Roth were in on a -- a -- big deal together. And that there was something in it for me if I'd help 'em out. He said that -- he said that -- you were bein' tough on the negotiations. But if they could get a little help -- and close the deal fast -- it'd be good for the family."

"A little help"? Fredo wasn't part of the Havana deal and Michael had no reason to tell him anyrhing about it--he even kept Tom out of the negotiations. Same for Ola: Why would he need to tell Fredo anything more than that Roth and Michael were negotiating "a big deal"? Even if Fredo had overheard or inferred something about Michael and Havana, and if he put that together with Ola's tale, he still couold not have had any details that would have been useful to Roth. So, what kind of "help" could he have given Ola?