Both the film's date and George Reeves' death date are correct. As with all films in the Internet Movie Database, the year listed in a movie title is the year of release, not of production. The 1973 Superman feature was a compilation film edited out of old episodes of the TV series "Adventures of Superman" (1952-1957), starring George Reeves. This information is listed in the Movie Connections link for Superman (1973) and explains how George Reeves appeared in a Superman movie released 14 years after his death.
Of course this isn't the only case: it's not uncommon for actors to be featured in films released years after their death. Another often-quoted example is Bela Lugosi , who passed away in 1956 but still managed to star in Plan 9 from Outer Space (1959), which was filmed and released three years after his death (stock footage and a double were used to create his posthumous performance).
Another example: actor Jeffrey Hunter was the original lead in "The Cage", the pilot for the series "Star Trek", filmed in 1965; however the pilot was rejected by the network, another actor was picked as the main character for the show (William Shatner as Captain Kirk) and the rest, as they say, is history.
"The Cage" remained unreleased/unaired for more than 20 years until it finally premiered on Christmas Eve in 1988. That's why Jeffrey Hunter is listed as having appeared on "Star Trek" in 1988, even though he died in 1969.