One immediate issue is that conscientiousness, e.g. the ability to maintain a difficult diet for an extensive period of time, is quite correlated with longevity

One immediate issue is that conscientiousness, e.g. the ability to maintain a difficult diet for an extensive period of time, is quite correlated with longevity. This is a very commonly known effect, which makes me more than a bit suspicious of this particular study. This is also why a control group with random assignment is necessary, particularly in the social sciences. There are simply too many confounding factors to make epidemiology useful in basically any way.