Even if the checklist was responsible for all of the improvement in surgical outcomes that doesn't mean the other items don't have an affect anywhere else. And

Even if the checklist was responsible for all of the improvement in surgical outcomes that doesn't mean the other items don't have an affect anywhere else. And again, it's not hand-waving to suggest that the item that has the largest direct impact on surgical operations is mainly responsible for the reduction. There's a difference between you wanting a more thorough study and something being "obviously nonsense".