It is a stretch. There are proteins that are built up in the brains of people with Alzheimer's and alcohol doesn't cause that. If there were a definitive link it would have been relatively easy to prove. No one with any credibility would say "long term excessive drinking = you'll get Alzheimer's"
Alcoholism has more to do with draining the brain of important metabolites as far as I can tell. The only link between the lack of these metabolites and Alzheimer's that I've seen is that some possibly bind and transport the bad proteins out of the brain. That puts Alzheimer's before alcohol consumption, not after.
On top of that, the correlation between the two would be as easy to prove as they come. They have not.