vegans are more likely to be 'rule followers' and engage in other lifestyle behaviors that are considered healthy (e.g., more walks, exercise, less processed foods, generally taking care of oneself). their positive health outcomes may or may not be related to their diet.

it has been shown repeatedly that you cannot control for these confounding lifestyle factors - hence why epidemiology should be step 1, and then you use randomized controlled trials to actually test for only changes in diet (which is obviously very hard to do at any sort of scale over any long time horizon - can't really lock people away for decades).

additionally, comparing a vegan diet consisting of whole, fresh foods to the standard american diet isn't really telling. anything is better than that!