Yep Jimmy makes himself look better just like Gravano did and all the others do as well. It makes them feel better about turning, it's just a human reaction.

As for him talking about Blackie taking out Nardi, Jimmy was floating around Cleveland according to the FBI IIRC around that same time. I think he had more to do with those bombings than he lets one believe. Most want to discount him like Gravano because they had so much info.