I don't think it's the ratio of dilution so much as the volume over time. Weaker beverages like 2% beer are less harmful than spirits because it's much harder to binge but they still do some damage. I don't think they are arguing for prohibition, their emphasise is on harm reduction. A world without alcohol might not be desirable and if we are informed about how to use alcohol in a sustainable way that is surely better than what we have now.