Part of it seems to be the lifestyle choices that smarter people tend to make: namely, that they smoke at much lower rates. Similarly, smarter people are more likely to follow other healthy practices, have a better handle on their health care, and be less likely to work in a job that puts them at physical risk.
Socioeconomic status could play a role as well, perhaps allowing people to access better education and care
There are also genetic factors.
Otherwise, you need to make the argument that IQ enables people to make better choices, but until recently in history human biology was a mystery to even the most educated. The famous "food pyramid", for example, promoted a high-carb diet and did not favour health or longevity.
Such underlying factors could be caused by random conditions; illnesses, environment, sleep habits, or very early in life - even in womb?
Outside of that, and outside of COVID, the third leading cause of death for many years was "Accidental Self-Inflicted Injury." Just below Cancer and Heart Disease. The presumed correlations between higher intelligence and avoiding becoming a part of these categories seems like another accidental measure here.
Genetic factors that affect intelligence may affect the whole nervous system. We already know that IQ is correlates strongly with reaction speed. It would not be surprising if there is correlation with immune system (immune and nervous systems communicate) for example.
pre- and postnatal health is big factor in decreasing intelligence. Parasite load and intelligence correlate in country by country comparisons.
same reason why married men live longer - less dangerous activities combined with wives who wanna keep their husbands alive so nag them about their health and prevention, for instance reason why my inlaw had to stop smoking
edit: also I will save you a click
"The slight benefit to longevity from higher intelligence seems to increase all the way up the intelligence scale, so that very smart people live longer than smart people, who live longer than averagely intelligent people, and so on."
"Part of it seems to be the lifestyle choices that smarter people tend to make: namely, that they smoke at much lower rates. Similarly, smarter people are more likely to follow other healthy practices, have a better handle on their health care, and be less likely to work in a job that puts them at physical risk."
Obviously if your work is less dangerous you are less likely to die, you could just get better work than improving your IQ. Same with prevention, you can have pretty low IQ if you just follow list of precautions, so only thing making the difference with IQ is those things you should be aware and it has nothing to do really in the end IQ. I'd like to see low IQ group vs high IQ group with same guidance, nagging about dangers of smoking, nagging about preventive checks etc, then we would most likely see there is hardly any difference. Maybe good test groups would be rich vs poor families taking care of kids requiring attention into late age like down syndrome or similar, them you work compare their health condition at same age, though obviously rich (read higher IQ usually) can provide better health, food and other options.
Japan and Italy are the countries with the oldest people. Both countries are surrounded by sea.
Maybe my IQ is not that high but this study of 1000 people looks off to me.