The cohort of non Jehovah’s Witness patients was vastly larger than the Jehovah’s Witness cohort and I can tell you now that patients with potentially the worst outcomes, who won’t take blood, the doctors won’t touch with a barge pole. The cohort of JW patients will be generally fairly healthy relative to many of the other patients. Not to mention the JW patients will have had far better pre surgical care, which in and of itself will lead to better outcomes.
It may be that in the instance of elective surgery, avoiding blood transfusions and or keeping the heart beating (in the case of heart surgery) does indeed lead to better outcomes. But that doesn’t negate the fact that if you need blood due to any number of reasons including trauma and cancer nothing but blood will do.