Incorrectly regarded as a mistake. Superman did not create a working automobile. Much like Superman was sent thousands of years into the future, so were multiple vehicles and assorted debris from the battle.
Vandal Savage could have sent a recorded message to his past self instructing himself not to follow through with the invention that would wipe out humanity. He did this when the Justice League had to intervene in World War II to make sure the Allied Powers won in "The Savage Time".