Actually Rugby union requires an enormous amount of brainpower from a few players on the field. There is a great emphasis on a struggle for possession and territory, and as a result the fly half and scrum half need to be able to sense when to run, when to kick, when to spread the ball, when to send a few phases up the middle. Due to the less continuous nature of the game, I think that these players would do much more thinking than any AFL player. AFL is i) a very impulsive sport where players have such a short time to sum things up ii) An egalitarian sport that doesn't have the specialization of other sports.
I guess it is much the same way that you would say a NFL quarterback needs to think much more than just about any player in any sport, yet other players on an NFL team don't have to think at all