It's not a well known joke, nor are you missing any nuance of the English language. Presumably Andrew was alluding to an earlier meta thread of his - [How come I never see any Aussies on here?][1], where he put a humorous slant on a genuine question. 

This would be known as an [in-joke][2] - a private joke that can only be understood by a limited group of people who have a special knowledge of something that is referred to in the joke.


  [1]: http://meta.ell.stackexchange.com/questions/3389/how-come-i-never-see-any-aussies-on-here
  [2]: http://dictionary.cambridge.org/dictionary/english/in-joke