The main aim seems to be to refute previous "U-shaped" and "J-shaped" studies that suggested a moderate amount of drinking was good because there was a dip in the distribution. Going so far as to suggest that moderate drinking must somehow be 'protective'. The explanation for that seems to be that those studies collected _current_ drinking use only, when presumably a history of binge drinking would still be quite relevant. This would artificially inflate the 'non-drinking' category with people who actually did have a history of drinking, while also deflating the moderate category. Apparently to the point that the moderate drinking levels looked even safer than non-drinking - which probably should've been a clue. In other words the data was probably pretty flawed, and garbage-in garbage-out.
As for this study...
"Genetically-predicted drinks per week" - are we serious with this?? Maybe they're claiming that their 'predicted models' align well with the smaller amount of surveyed self-reported data but that's hard to find in the paper.
They seem to bend over backwards to make alcohol causal, even going so far as to suggest that a decline in drinking behavior over the years may just be reverse-caused by the future dementia. And for higher incidence of dementia in non-drinkers - seemingly the opposite of the conclusion - that's explained away by suggesting those people may have just had a hypothetical prior heavy use, therefore the "reverse causation is further supported". Pretty circular...
I'm not sure how much more can be reasonably concluded from this other than health risks probably scale with drug use in some fashion. The data, methodology, and modeling seem far far too hand-wavy to suggest any kind of definitive explanation. The results barely even exclude 'no effect' in a 95% CI.
I would not be surprised for a second if effects like early cognitive decline correlate with decreased drinking habits, but I just don't see how you can conclude any of that from this.
I suppose if we're getting off the apparently very loosely suggested 'moderate drinking is good' myth, that's still progress, but...