Or there could be different explanations for different sub-cohorts. People predisposed to depressive symptoms might also be in a group of those that rely on moderate drinking for emotional regulation, AND a possibly separate group of people may have a kind of extraverted social life where social drinking is commonplace, and thus after birth suffer a significant lifestyle change which may trigger depressive simptomatology.

I'd hazard a guess it's easier to adapt to the "4th trimester" if one is a settled homebody vs if one is a party-hard person. The party-hard person may also experience dwindling of their support network because their old social circle is less baby-friendly, etc.

Like this is just an expected cluster of co-occuring phenomena with likely catch-22 mechanisms involved; of course all of this correlates.

It also most likely does not mean "ditch all the alcohol during pregnancy, keep behaving the same otherwise, and you're significantly reducing your chances of post-partum depression", which would be the meat of it if it did, but.

These papers always pretend like they're saying something more useful or meaningful than they are. There's no real harm reduction plan or initiative you could make off of this finding that's not already covered by common sense and/or FASD initiatives.