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?, where he put a humorous slant on a genuine question.
This would be known as an in-joke - 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.
