"A beer? You drink alcohol?"
Translation:Une bière ? Tu bois de l'alcool ?
Coming from a general English sentence "Do you drink alcohol?" how is the quantity a matter contention at all? is it not, do you drink alcohol in general, the premise of this question?
Right. The definite article, not partitive article, should be used because the speaker is asking if the other person drinks alcohol in general, not whether they are drinking some alcohol.
I don't think it is the same as you indicated. In my opinion, if we use l'alcool here, it means specific alcohol, not generality, we can't drink all the alcohol on earth, if I do not wrongly interpret.
Question: Is it "de l'alcool?" because it's not one glass of alcohol but a generality?
Correct. "boire de l'alcool" when the amount is not defined and "boire un verre". There's a good summary on Definite/Indefinite articles here: http://www.languageguide.org/french/grammar/articles/
This example is partitive ("some") for the alcohol bit, for some reason other comments seem to be saying this is definitive. The confusion may be that this is not "de la" or "du" however when the noun starts with a vowel, whether masculine or feminine, "de l'" is used (de l'alcool) for the partitive.