They never said it was Sicilian Mafia.
"The Five Families - the Bonanno, Colombo, Gambino, Genovese and Lucchese - have ruled the city's Italian American mafia since 1931."
"The Five Families are part of the larger American-Sicilian mafia operation known as La Cosa Nostra"
It's being said because the Five Families was erroneously conflated with Cosa Nostra in the original press conference by one of the representatives of law enforcement.