This is a terrible study design. A person's physical fitness doesn't remain the same over time. The study looked at health at age 18 and then locked in the groups for the rest of the study. A doctor is going to say, "Exercise now," not "Go back in time to age 17 and exercise so you have good fitness at age 18." What matters is how exercise impacts a person over time, not how exercise from 30, 40, or 50 years ago impacts you today.
Fitness
I don't think this was the purpose of the study, but the article is misleading: it was a response to the statement that good physical fitness in youth has many health benefits (regardless of actual fitness). The results of the study suggest that it is very complicated to set up comparable groups, because if random mortality (i.e. with no apparent link to fitness in youth) is lower in the group that was active in its youth, it is because some parameter (e.g. socio-economic status, personality) has not been taken into account. Previous statements about the importance of fitness in youth may therefore have been overestimated by this parameter.
The study didn't even give honest comparable groups. It treats people born in 1954 the same as people born in 1977, even though things like BMI, amount of youth physical activity, and quality of medical care has not remained the same over time. Actual mortality is much more likely to have happened to people born in 1954 than 1977 and so the people in this year weigh more heavily vs. the total study population. With so many uncontrolled variables, I don't think this study can tell us anything useful. In my honest opinion, any correlation between accident mortality and other types of mortality studied could be explained by correlations that are due to one of these uncontrolled variables and not the object of the study.
I gave this study some more thought today and I think the authors are making another big mistake. Let's say two people fell off a two-story roof while cleaning gutters. They have an identical fall. They are both the same age, say 60. Who is more likely to die as a result of that injury? The very out-of-shape unhealthy person or the healthy person? The g-forces when they hit the ground are different. And their ability to recover are different. Yes, healthy people should recover from accidents more frequently than unhealthy people.