Although the study is not an RCT (how would we design one?), I find it convincing that cognitive ability is partially responsible for, e.g., educational attainment, even if this effect is often adulterated by other factors like parental income.
The paper focused on being "too smart." On the other side of the spectrum are folks with dementia. Those who experienced dementia in their lives can attest to how devastating of a condition it is.
If a decrease in cognitive abilities leads to impaired independence (and consequently life outcomes), then it would make sense that an increase in cognitive abilities leads to improved life outcomes. Here, I am assuming that this effect is monotonic across the whole range of cognitive abilities, which the submitted article seems to confirm.
Of course, I am comparing individuals having a medical condition (dementia) with, as far as I understand, cognitively intact study participants, but I think it doesn't meaningfully weaken my argument. If you disagree, I'd love to understand why.
This is exactly why.
Because dementia has specific non-global effects on cognitive ability. It's a disease. The impairments caused by dementia aren't a linear projection down from a non-diseased state. Even if outcomes in terms of how the study defined them end up in a similar place, dementia isn't actually supporting evidence in any meaningful way.