I know about local variation. I live in one of those places where the average male life expectancy is over 80. I confidently expect this to drop when we leave the EU. I am also aware that there are subway lines in both NY and London where life expectancy varies by around 7 years from one end to the other.
Also, different populations tolerate different diets to a degree. I believe the original research on salt levels was done on black Americans, but I can’t find the citation. There are reasons why they might have experienced population selection. Unless equivalent studies have been carried out on other ethnic groups, they shouldn’t be generalised. (Consider the very different patterns of tolerance to alcohol shown by Han Chinese, Siberians and Western European as examples along with lactose intolerance.)
