You can't go around saying a group of people are a certain way without actually understanding them in the same way you can't fix a computer if you know nothing about them.
All autism research is vauge at the moment so the books you talked about could easily be wrong, there are probably other books which say the complete opposite.
Psychologists and Psychiatrists specifically study people. They don't have to be one of a particular type to study them.
What they are doing is saying this: whatever the incidence of alcoholism is in the entire human population, it is higher in the specific group who are on the Autism spectrum. This says nothing about any one person, but rather the entire group as a whole.
I don't think it's surprising at all. Considering the challenges faced, it's a lonely difficult road to travel. My son is autistic, and that has changed just about every aspect of my own life. He's on a restricted diet, so if we're out and about, and get hungry, if we haven't brought lunch, we go hungry until we get home. Trying to get gluten and dairy free foods at a regular restaurant is just about impossible.