It's already been shown that being social, being physical either just walking or dancing or similar, and doing mentally stimulating stuff like reading and such is correlated with lower chance of dementia (or at least older onset).

The above activities seem to me more likely to be performed by someone who has an optimistic spirit. If you think walking outside will be just fine, you're more likely to do so etc.

So the results doesn't seem that surprising to me at first glance at least.