The study covered ten years and none had dementia at the start.
My somewhat sarcastic point is that correlation does not prove causation. This is a common problem with many ‘studies’. In addition, as among the ‘aged’ myself, I’m well aware that not having any outward signs of impending dementia doesn’t mean that it hasn’t started. After spending many decades of ‘close calls’, I’m far more cautious in doing almost everything, than I was when I was 20. What’s the difference between caution and dimentia?