I agree with you that the answer is quite generic. Altho Google is able to know where the traffic is referred from. Via HTTP Headers you could already easily find out where a page will refer to, like I did with this case when using a 301 redirect. While Google isn't probably using this option it is possible as you see.
I especially like your reference, because a couple of months later the guy who replied invented the wheel again and is implying in this article the opposite in my opinion.
To get some more information about your question, did you sent out recently some email campaigns or started up some SEA/ PPC campaigns?