well if you sampled the 6 billion people in the world, yes, rich people drink alcohol.
I think its a bit like saying 'rich people drive Ferraris' - yes, of course! having a lot of disposable income will obviously lead to the ability to purchase luxury, non-essential items like alcohol.
That is an interesting statement. I'd estimate that the sample would show that alcohol was used by people with a broad range of income levels. However, the only way a hunter-gatherer (as described by Diamond) would obtain alcohol is to trade with a farmer or someone who can do the meta-work required for alcohol and supported by farming.
I think its a bit like saying 'rich people drive Ferraris' - yes, of course! having a lot of disposable income will obviously lead to the ability to purchase luxury, non-essential items like alcohol.