This is exactly why it's difficult to properly control for the supposed moderate drinking that's "healthy". Sooo many ways to explain why "moderate" drinkers are healthier:
1. Maybe they're a self selected group of people who can properly moderate usage of something that's bad in order to get the benefits, in this case, friendships and social activity.
2. Maybe drinking is bad and moderate drinkers know it. Therefore moderate drinkers intentionally moderate their drinking and overcompensate in other miscellaneous healthy activity.
etc etc etc