AA is spiritual treatment, and any attempts to study its efficacy in treating alcohol-related problems should classify it into the same category as Christianity, Islam, etc,, since you cant really "study" the steps. They arent quantifiable. Unlike a "pill", or a set of instructions where if you follow A-then B happens, AA and other spiritual treatments aren't like that. I'm hoping I make sense.
I'm not saying there's anything wrong with spiritual treatment, and in societies where there is a strict religious taboo towards abusing alcohol-there arent many alcoholics! So of course people with experience in religious settings do better in AA.