I read this, and remember thinking that it must be complete bullshit. (and also wondering how long it would take someone on THP to pick it up) Why, specifically, is the dementia greater in those who are thin? Could it be a secondary factor? Immediately, smoking comes to mind. Let's face it... Most people - especially in this country - don't tend to get slimmer as they get older. Also, people who are "thinner", in general, tend to fit other profiles. Blood type, medical history, etc...
I have to hope and believe that this is a "gateway" study. One done so superficially, that it invites further research.
Also, the "lower income" finding is SIGNIFICANT.