I'm not to sure if the game still uses the original framework, not even sure if A1 was started on unity, but I do remember reading during my research Joel bought the rights to the voxel engine/framework that the game is/was made with. Its been a few years so my memory is a little rusty. I also forget the name of the guy Joel bought the engine from....
EDIT: I am not familair with these terminologies, forgive me if I'm using the wrong terms lol.