Compared to the paragraph before it, I think he's just saying our estimates of ABV may be wildly off because we don't have a great historical basis for them. Ou

Compared to the paragraph before it, I think he's just saying our estimates of ABV may be wildly off because we don't have a great historical basis for them. Our modern notion of what constitutes wine or liquor is based in part on ABV ranges. If we say "ancient peoples drank wine nonstop", someone might think you're talking about a ~15% ABV beverage, when that isn't necessarily true.