Alcohol may have 0 carbs, protein, or fat, but that doesn't mean it's calorie free or diet friendly. Pure alcohol has 7 calories per gram. Moreover, our bodies process alcohol first, so any fat, protein, or carbs that are present will be stored until the alcohol is gone. This may slow fat loss.
Plus there's the munchie effect -- people tend to go off plan when they drink. Others retain water. Either way, it attenuates the weight loss people see in phase 1.