I reached the same conclusion in that other threat TB, and would add some more observations.

After Fredo arrives with the money, Michael tells Fredo that Roth is going to assassinate him. During that conversation Fredo ruefully says "why didn't we spend time like this before" and "I was so mad at you Mikey." Later that day Roth is far more agitated than he had been. He tells Michael that he is aware that the money reached Cuba. I suspect Fredo, after spending time with Michael contacted Ola or Roth to tell them the money had arrived AND that Michael was on to their assassination plot. Furious, Roth explodes in an uncharacteristic way when Michael pushes him on "Who gave the order" to kill Pentangeli. By this point Roth probably knows his assassination plan isn't going to work, and he may have also known that all the money he was collecting allegedly to pay off Batista may have been destined for his pockets all along. Note how distressed Roth gets when Michael publically suggests the rebels could win. I'm thinking Roth knew the rebels could and even would win, and was planning to pocket the money himself, claiming Batista had taken it. Now, knowing that he didn't have a partner in Michael, he secured his arrangements to leave Cuba.
It is entirely possible that his trip to the hspital was a ruse to save himself from being killed. Remember that Michael's body guard had to settle for killing Ola at the hotel, and had to rearrange his plans to get Roth at the Hospital, where Batista had disatched some troops to guard him, and who arrived in a nick of time, and who probably spirited him away.

Fredo had to have been aware all along that Michael would be assassinated in Cuba, and he was probably aware of where to go if he needed to get off the islad in short order....so yes, I believe Fredo and Roth were on the same boat to Miami.

No question they Roth camp was using Fredo as a mole, and despite Fredo's trepidations at one point ...the telephone scene where he says they lied to him and tells Ola not to call him anymore...he stayed with the Roth people in an effort to rub out Michael.

Assuming all this to be true, once again we need to wag our finger at Tom Hagen and somewhat to Michael who never had a clue this was going on. From the time Fredo called out Michael in front of Moe Green (a staunch ally of Roth, and the person who most likely introduced Fredo to Ola and Roth) somebody should have been watching Fredo. No way should his meeting in Beverly Hills have gone unnoticed.