I know of no reason why diversity alone would lower longevity. Differences in genetics aren’t that significant.
It is tied to many things but genetics being one of them. The point being white people live similar ages in similar countries.
I would venture that genetics has little to do with that. We know that people in Japan live a long time, but I doubt genetics is a major factor. They have a country with low rates of violence, a diet with less meat and sugar and more vegetables, more soy. Could genetics be a factor as well? Maybe, but probably a small one compared to the behavioral/cultural ones.