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The sentence sounded weird to me too and I interpreted it as a non-native english speaker simply saying their friends drank alcohol but where not alcoholics. OP should probably clarify.

In the context of 23 year-olds, I think it's safe to say that none of them are actual alcoholics, just that they drink A LOT, as many people at that age do.

How is that safe to say? There are plenty of 23-year-old alcoholics, having lost control over their alcohol consumption, in full-blown physical and/or psychological addiction. You don't have to age into it, or take a test, or carry a card. It just happens to you.

Yes, but using the singular signals a non-native speaker, and this is reinforced by using "couldn't" rather than "haven't [been able to]" in the third sentence. I think GP is on to something.

I think you're reading too much into it there. Plenty of native speakers have far worse grammar and more questionable usage choices, especially the ones that are young and drunk.