Earlier quoted context omitted.
That the lifestyle difference is irrelevant since any study that measures the impact of a food in health should control for it.
It can't be controlled for if there is no variation in the population. At that point it becomes a fixed effect--a population-specific intercept, as it were--and the only way to net that out would be through a difference-in-difference type study.
You think there are no Tibetans who don't exercise much and don't eat much vegetables either?