Oli I think he used it loosely to show he was truly sincere, but I think at that point he knew Michael was going to take over and he knew (I think) in his mind that Michael would get revenge because they killed Sonny and of course tried to kill Vito. By saying that, he knew they would eat it up as Barzini thought by Vito giving in that gave him more power and it also brought drugs in like they wanted with no war.