From the conclusion of the study the article cites:

> In this survey study, all types of objectively measured VI were associated with a higher dementia prevalence. As most VI is preventable, prioritizing vision health may be important for optimizing cognitive function.

In other words, the association is dementia and uncorrected visual impairment.

EDIT: I'll add that the study does not show causality. Even people like myself who are well aware that correlation != causality often still assume it does. It doesn't help that the authors themselves imply that correcting VI would ward off dementia. It might; it might not. People with dementia might just be less likely to get glasses.