You don't get his point.
He's saying that if you have extensions hidden, then you will only see "partyinvite" and you will never see "partyinvite.doc". So if you see ".doc", you should know that something is wrong.
(As a side note, how does anyone in Mac or a default Windows installation know that something is an application and not a document? In both cases, the extension is hidden, so the user has to look at additional information provided by shell. Looking at the extension in this case is simply not reliable since they don't even show up. In addition, both Mac OS and Windows now provide a warning dialogue the first time you run an application to warn you that it is executable. So I really don't see how the problem cited in this article should be considered a vulnerability.)
Never call a man a fool. Borrow from him.