Brett is right on the money. That passage from the book says it all. We all know Scott is an addict, and right now, alcohol is his drug of choice. (And it is a particularly destructive one to be addicted to.)
I applaud Brett for calling this what it really is: a relapse. Scott is an addict, one who happens to be a famous and incredibly talented musician, but still an addict who has relapsed on alcohol. He is special for his talents and the music he has created, but he does not possess some unique ability to continue drinking without it affecting his life and career in a negative way. If you disagree, there's a long list of other famous and talented musicians who destroyed their careers and/or died too young because they thought they were different in this regard too.
Addicts can't drink. They just can't. It's a cardinal rule of recovery. As long as he keeps drinking, he's not going to get any better, and the best we could hope for is that he deteriorates slowly, not quickly. I hate to be blunt, because I love the guy as much as everyone here, but this is the reality of it.