I've always wondered if this simply isn't a case of having the causality wrong, that is to say people in old age who don't have alzheimer symptoms are more intellectually engaged.
In fact, any question you have that relies on thousands of doctors and professors in the field missing something obvious is always "no" (cf "never a compiler error").
Are you scoffing at the idea that Alzheimer's symptoms could reduce intellectual engagement?
Or just at the idea that experts could have missed it in favor of the opposite relationship?