Separately from this study, here's an interesting opinion piece by John Ioannidis titled "The Challenge of Reforming Nutritional Epidemiologic Research", published in JAMA 2018:

https://statmodeling.stat.columbia.edu/wp-content/uploads/20...

  > Assuming the meta-analyzed evidence from cohort
  > studies represents life span–long causal associations, for
  > a baseline life expectancy of 80 years, eating 12 hazelnuts
  > daily (1 oz) would prolong life by 12 years (ie, 1 year per
  > hazelnut), drinking 3 cups of coffee daily would achieve
  > a similar gain of 12 extra years, and eating a single man-
  > darin orange daily (80 g) would add 5 years of life. Con-
  > versely, consuming 1 egg daily would reduce life expec-
  > tancy by 6 years, and eating 2 slices of bacon (30 g) daily
  > would shorten life by a decade, an effect worse than
  > smoking. Could these results possibly be true?

Studies of larger populations yield more typical results. Consequently, studies of smaller populations yield more extreme results.

That's not to say that these results might not be significant -- what you propose may be the case -- but I'd want to see an actual mechanism of action before buying something like this.

Idk about the op study, but I could imagine confounders with instant coffee consumption.

No, it affects everybody. Says so in the article. The distinction appears to be that severe mental illness is associated with shortened lifespan so coffee has a more profound anti-aging affect on that population.