Well, that whole scene is a bit curious, dt:

When Michael asked Tom where Fredo was, he replied: "I think he must be in New York." You or I would have immediately asked: "You think he's in New York? How do you know? Did he call you? What did he say?" But Michael didn't. All he said was for Tom to get word to Fredo--confirming that he accepted that Tom knew where Fredo was.

Another anomaly: Michael was frantic to get Fredo on the plane with him out of Havana. And, the first thing he hammered Tom on when they were alone was, "Where's Fredo?" Michael could have arranged for Fredo to be snatched from whereever he was hiding out, and brought to him. But he didn't. After that scene in the Vegas hotel room, the fire seemed to have gone out from under Michael's urgent interest in Fredo.

Perhaps you're right: Michael might have been slippin'. If he'd grabbed Fredo immediately, Fredo might have told him about Pengangeli's survival and Questadt's beloning to Roth.