I think that Spinelli couldn't find out who Dante was. Dominic Pirelli probably just starts and stops in records somewhere. Then in the police records they were simply referring to the evidence handed in as coming from an undercover cop. Yet, for a mob boss responsible for some pretty delicate decisions Sonny isn't very analytical or logical. He's just shot a man who's still in the hospital and he's at Jason's penthouse ready to shoot another one. Not a great learning curve.