In fact, it says that decaf coffee also has that association, so that would mean it's not the caffeine.

It also says they don't make the same conclusion about artificially sweetened coffee (the study says it's "less consistent"—seems I need to pass the paywall to see the numbers on that). One wonders if they believe that the artificial sweeteners are killing people, or that they interfere with the benefits of coffee, or if they'd say this reveals that selection effects are strong here.