The most likely explanation is that he had a Great Engineer either already in his city, or it came available in those 2 turns. You can sacrifice your Great Engineer to complete a World Wonder in 1 turn (up untill a certain point, after that it will just add a lot of hammers so it will decrease the amount of time to build the wonder dramatically). A lot of People try to 'force' a Great Engineer by building the Pyramids (gives you GE points), Forge (makes it possible you can assign one of your population as Specialist Engineer), and sometimes even The Hanging Gardens (also gives GE points). The idea is that the chance of getting a GE is pretty high this way, and typically the GE is used to rushbuild the Great Library (although you can use him to build any wonder).