I never thought about Turnbull's point that this line was said for Ritchie the bartender, in case he ever had to give someone up.
His theory doesn't hold water. The bartender wouldn't have cooperated with the cops (and give testimony against Michael Corleone).
The line was delivered (IMO) because of a similar line made against Larry Gallo (and it was somewhat common knowledge) and FFC heard Aiello ad-lib the line in rehearsal and agreed to keep it in the movie because it sounded authentic.
