cant deny that the 7d is extreamly versitile, might not have used all the features of it in this video but im sure there will be a time where he is very glad he has such ability. For DOF its hard to use a system like a redrock because it limits the light so much, and it just looks like a bitch to use imo. put ontop of that that its the same price, why not sell the red rock he had to get the far more run and gun 7d(not saying in this case he used dof that much, or that he sold the redrock, did you?). Also im guessing a housing is wayy cheaper for a 7d then it is for an hpx so thats one reason to use that camera for this edit where the hpx wouldnt have worked.
I know i put in alot of assumptions in this post and we all know what assuming does