Yes, and since there is a factor that affects both fitness and dying in car accidents or homicides, then it's harder to establish for how big reduction of morta

Yes, and since there is a factor that affects both fitness and dying in car accidents or homicides, then it's harder to establish for how big reduction of mortality is fitness really responsible (causal). What you are complaining about is exactly the point of this study: > This suggests that people with high and low fitness levels may differ in other important ways, which is something that previous studies have not fully taken into account