They've found correlation and support this with studies finding that isolation has harmful effects.

I think it would be much stronger to conclude that the traits which cause a person to be capable of and want a long-term marriage are healthy traits which reduce dementia.

Here's my personal takeaway... You know the least helpful, most anti-social, least interested in long term commitments, and least healthy people in the world? A lot of them aren't married and don't have children! People like this also develop dementia! Why do they develop dementia? Well, probably some combination of what they are missing because they lack social interaction or because of other traits that lead them not to have much social interaction. We aren't sure which is more impactful.

Anyway, the study doesn't claim causation, and backs up a causal theory using related research. They also use language that implies their theory is only one of many. But in my personal opinion, they are still treating it too much like it's causal data and like they haven't identified other theories or suggested future resources to help create evidence for one theory over another.

To me this raises important questions. Does the benefit to the mind come from social interaction or something specific to marriage? Or something specific to minds that are capable of quality social interaction? Does being in a marriage increase this capability significantly, or does the drop-off in this ability just cause marriages to disintegrate?