I don't know who they studied, but a huge group of non-coffee drinkers that does get occasionally studied is members of The Church of Jesus Christ of Latter-Day Saints. No coffee or tea but also no alcoholic beverages, tobacco or street drugs either, and otherwise whatever foods/diet an individual wants. With alcohol and cigarettes absent, as you can imagine health measures are statistically better. But a fair study of coffee or tea would require finding a comparison group of people who also don't smoke or drink, and I don't know who all that might be.