It may be. But early on, the Wallabies seemed to also play an attack structure with 11/14 on one side and 12/13 on the other. Kind of like a league centre/wing thing. Kerevi was on the wing then, as well.
I think that Cheika missed a step not subbing Sio a few minutes earlier there, he was clearly struggling at scrum time and Slipper should have been ready to go after that dropped ball.
Sio is still a very good prop, but he's been rather out of form this year for the Brumbies. I was surprised that he started ahead of Slipper as Slipper has been playing a lot better, but I've also got no problem with Cheika having faith in his WC players.