Duel is a 1971 television (and later full-length theatrical) thriller film written by Richard Matheson, which is based on his own short story. The film is the full-length film directing debut of American director, producer, and screenwriter Steven Spielberg.
Steven Spielberg produced directed more movies after Duel. 
The statement says it was his debut film. Usually when it says debut it means that was his first and he went on to do more. 