My mother weighed less than she ever had in the time I knew her when she was diagnosed, part of the reason for my concern about her health was her weight loss, hence I took her to the doctors at which time I expressed concerns about her memory issues and the tests started and it lead to a diagnosis of AZ.
Likewise years later with my wife she went down from a dress size 12-14 (and a couple of size 16's in the wardrobe too (don't tell her I told you that bit) and eventually when we got it through to the doctors something was wrong and to stop fobbing it of as "women of a certain age" issues she weighed in at 6st 4lb.
Unless they've considered whole life average weight and not just at the time of diagnosis then it would be a bit meaningless, maybe they did I can't see anything either way.
Until someone explains the science then all it is at the moment is a coincidence and as I've said before coincidence doesn't prove causality. Italians eat a lot of pasta, Italy has a problem with organised crime, therefore eating pasta caused organised crime.
Based on my own observations (so totally none scientific) I would say those in the lighter end of the weight scale do seem to be the vast majority of those I see with AZ and there is a noticeable lack of the very overweight compared to the general demographic, but maybe they have mobility issues so don't go to the day centres.
K