Giuliani is not a Federal employee, he is one of the President's personal attorneys.
Giuliani can say whatever he wants in public.
Moreover, what Rudy is doing is preparing the public for what is soon coming.
What needs to be kept in mind is that before he was mayor of NY, Giuliani was the United States Attorney for the Southern District of New York. He took down the leadership of all of the 5 Families of the mafia.
He knows a thing or two about prosecutions and indictments.