Interesting answer. I'd say that being an extrovert (which I am by the way) is not a protection against social anxiety. To give an easy example: every male friend I ever had, extrovert or not, had to drink some amount of alcohol to have the courage to flirt in social environments. But business meetings, family meetings, etc, are also nerve-wracking.
Alas, the addictive nature of alcohol obviously plays a role as well.
Additionally, alcohol is an addictive substance. I know plenty of extroverts who don't need any booze to socialize and still very much enjoy drinking.