Si Superman 2 Donner's Cut continúa una versión de la primera película cuando el viaje en el tiempo no ocurrió, entonces técnicamente Superman: The Movie: Extended Cut no es canon para esa línea de tiempo.
Basically, they were working on both Superman: The Movie and Superman II at the same time, and the ending of Superman II was supposed to have the time travel. Lester convinced Donner to leave production on Superman II for later in case Superman: The Movie was a failure. Additionally, they wanted a dramatic ending rather than a cliffhanger, so they borrowed the original Superman II ending with the idea that they would come up with a different ending for Superman II.
Superman II: The Richard Donner Cut restores the cliffhanger and the original ending, thus taking from the original Superman: The Movie script, thus negating the time travel.