Well gee, I guess you're right. If only there were some sort of test to determine if a person had been drinking, and how much alcohol they'd had. Someone should get right on that.
Certainly a binge-abuser can get a hold of alcohol on the black market at a certain price, but in general, if Islam is established somewhere such that the corner liquor stores don't carry liquor at all, how many people will be alcoholics on the D.L. anyway?
If alcohol can't be had on the market under normal circumstances, is it not reasonable and rational to presume that most subjects would not be clandestine binge-alcoholics who hide it so well that weekly testing won't catch it? I mean, really.
I mean, sure, you can administer a breathaliser when they walk into their monthly appointment with the staff who're surveying them, but that's hardly going to detect a glass of wine they had last Saturday.
One can, for example, imagine a world where a culture of covert hidden drinking in specific Muslim countries gives the proposed benefits of moderate drinking to some of the population. A study that assumed no Muslims drank would thus be flawed because it would include a group that actually was drinking all along.