““It’s a little bit like granting immunity to Bonnie to get a Clyde,” Schweizer said.”
No. Bonnie and Clyde were in the same gang. It’s more like giving Al Capone immunity to get Bugs Moran. They were adversaries and Al would have been happy to hurt Bugs anyway. Just like Podesta would be happy to hurt Trump, or Manafort if he can’t get to Trump.
Why would anyone believe Podesta given the circumstances?
Well, since we're talking about the FBI, I will note that the FBI does exactly this. In Boston, the Italian Mafia was taken down by the Irish Mob -- Whitey Bulger had a very cozy relationship with the FBI and the FBI pretty much did whatever Whitey told them to do. Voila! Whitey's criminal competition got taken down almost completely.
FBI is beyond sleazy and has been for a long time.