From the article:
As predicted, studies with younger cohorts and separating former and occasional drinkers from abstainers estimated similar mortality risk for low-volume drinkers (RR = 0.98, 95% CI [0.87, 1.11]) as abstainers. Studies not meeting these quality criteria estimated significantly lower risk for low-volume drinkers (RR = 0.84, [0.79, 0.89]). In exploratory analyses, studies controlling for smoking and/or socioeconomic status had significantly reduced mortality risks for low-volume drinkers. However, mean RR estimates for low-volume drinkers in nonsmoking cohorts were above 1.0 (RR = 1.16, [0.91, 1.41]).
Studies with life-time selection biases may create misleading positive health associations. These biases pervade the field of alcohol epidemiology and can confuse communications about health risks. Future research should investigate whether smoking status mediates, moderates, or confounds alcohol-mortality risk relationships.
People that don’t drink used to drink a lot or had other conditions that stopped them from drinking. This wasn’t taken into account when creating studies about it not drinking, and low drinking where done. This made it look like low drinkers fared better than non-drinkers. But non-drinkers were non-drinkers for an unhealthy reason. And that’s why we ended up with studies that said some drinking is healthier than none. But it turns out they were wrong. Because the non-drinkers they used had medical issues that prevented them from drinking and caused more problems.