It depends how you look at it. The mission calls for both systems. Making success reliant on two completely different heavy lift vehicles simultaneously, in whi

It depends how you look at it. The mission calls for both systems. Making success reliant on two completely different heavy lift vehicles simultaneously, in which the mission fails if either fails, is arguably not de-risking. Yes if only one succeeds the US will still have one heavy lift system, but that would require massive re-vamping of NASA's planned mission goals.