You're correct in that AA says you can make anything your higher power. I was speaking for myself and my own approach to the higher power concept. You can use whatever you want as a higher power, or nothing at all if that's your belief. AA doesn't insist on having one, it merely suggests that. I'm not sure where you're going with the spiritual disease thing. AA is a spiritual program and alcoholism is a disease. And spitituality in the AA sense is merely your personal relationship with a higher power.
I'm also not familiar with anything that AA has ever said that indicates one is born with the disease of alcoholism. Some folks may feel that way. I don't. One may have a genetic predisposition for the disease, but only alcohol causes one to actually have it. And if you're predisposed yet never drink, then I don't think you're an alcoholic. But I'm gonna leave that one to the scientists. It really has nothing to do with recovery, and recovery is purpose of SR.