and to let us, the audience know who is the traitor in the family
I think the other half of the conversation is to let the audience know that Ola and Roth are the main adversaries. We're seeing one side of a chess game, and don't see much linking Ola-Roth to the shooters until we're very far along.
Besides even Fredo would have known there was no way he could operate outside the Corleone business, something on his own Fredo knew the only way “Fredo wouldn't be stuck running Mickey Mouse nightclubs” is if Michael was dead
I don't know how anyone can speak with this sort of certainty about what's going on - in particular what some character must be thinking. Fredo, in particular, is hard to pin down because there's no character continuity from the book to first movie, and very little continuity in GF2.
