Maybe A causes B. And sometimes maybe it's B causing A. And more likely much of the time, it's C that causes A and B separately, and they don't even mention C.
Applied to this example let me speculate: Intelligent people are more likely to plan on and go to college than people who recognize they are intellectually limited. That time in college may be worthless, but so long as it doesn't destroy their fundamental intelligence, they are still going to be more intelligent than less intelligent people who didn't go to college.
So the less intelligent people wind up in dead-end jobs, and start boozing, or doing drugs, to cope as they deal with a dull life, broken relationships, and so on. And that leads them to an early grave, particularly as government policies sap the dynamism and opportunity out of American culture. That's what I think I see happening. It's not that college somehow extends life significantly, but those who attend have better job opportunities either because of their native intelligence or the grifting network that a degree provides, that keeps them out of the tailspin of despair facing much of the population.
Not to disparage intelligence, but I think a strong family background is the backbone of a successful life and career. ( Your mileage may vary . )
I think perhaps only 25-30 percent of young people have the capacity that would enable them to truly benefit from higher education. The rest would be better off going to a technical school or entering some kind of apprenticeship.