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.
As I understand it, some studies don’t distinguish low-volume drinking from not drinking. Grouping non-drinkers with low-volume drinkers will bias that group to look healthier.
In addition to this, some studies don’t include younger individuals. I suppose this biases the study outcomes towards older-age related effects while neglecting younger-age effects. An example of an older-age effect could be an interaction between alcohol and a blood thinning medication or heart disease. An example of a younger-age effect could be excessive drinking combined with behaving like a complete buffoon
The quoted portion of the meta-study in the post makes it clear that the studies reviewed did distinguish between low-volume drinking and not drinking.
It wouldn’t surprise me if sample selection not taking into account social factors which would cause people who drink at low volumes to lie and say they don’t drink could play a role in certain studies.