Agreed re: the historical argument not being a great gameplay balance argument. But I do think it's worth exploring some historical motivation for how gameplay actually comes out. Obviously the game is not 100% historically accurate, but things should "make sense" on some level. In this case, I think they do in terms of training time. Riding an elephant is harder, but there are lots of other reasons that training the people who fight on elephants is easier. But feeding the big boi takes a lot of food, hence the high food cost.
I am not sure why I didn't respond to this earlier but you are spot on. Even if historical accuracy is not the sole determinant of design and balance decisions it is always a relevant consideration because it is the foundation on which the game is built.