Key is 'patients who routinely' - they didn't prescribe Crosswords and Chess - these are people already disposed to doing those things, which will be a subset of all people.

I remember a friend taking up tennis for a similar reason (he read that those who play tennis, live longer) - but the article negated to note all the things in a person's life that will set them apart if they are a regular tennis player - which are not caused by playing tennis (relative wealth, free time, ability to easily travel to the location of tennis courts, job/family life set up that allows for hobbies etc).

I didn't tell him to stop playing tennis, nor do a feel that keeping an active brain is pointless - especially as it can help highlight when changes do happen.