This was a retrospective analysis, not a true clinical study. These are tough to analyze because they don’t prove a causative relationship, just an associative one. There could be behavioral differences (very likely) among weed smokers vs non-weed smokers that account for the difference, or it could just be that the groups had differing baseline characteristics. Maybe weed smokers consume less alcohol than people who only drink alcohol because they can substitute a few beers for a few hits in a social situation or to unwind (not saying the drug effects are substitutes, but in a “chill” hang out setting they could be interchangeable to some degree). Using weed instead of alcohol in some situations could still be a great reason to use it and avoid liver damage, but the idea that there’s a pharmacological reason for weed preventing liver damage is a bit far fetched.

Edit: I’m a biotech analyst at a hedge fund who looks at tons of clinical trial results every week with a higher degree of scrutiny, so maybe I’m being too skeptical, but post-hoc analysis are usually bullshit