After the accounts that submerged in GFI, when you see Fredo start to "go his own way" and violate orders, and his treachery in GF2, it's quite obvious to the audience that we know Fredo would never be Underboss. Even if Michael named him successor upon his subsequent death, it would be a matter of time before Rocco or Neri took command of the Family. But to the FBI, that was about as credible and as close as they were going to get. My best assumption: maybe a little hast in the details with ranks and names, but excellent nontheless in displaying desperation and detective work on the part of the FBI for GF2.