I don't know how to explain it to you any clearer. The Tea party is ideologically far right. They are further right than any other group. That is what makes them extreme. That is how the article defines extreme. If there were other political parties that were further to the right, than you could attempt to make an argument that they were centrist, but there are no other groups further right. Just as a communist would be extreme left, the tea party is extreme right. Everyone else is in between.
The reason the article is so ridiculous is because it gives you a definition of what extreme is. The Tea Party, by his own definition, is clearly extreme, but then he tries to argue that they are centrist based on a few polls showing the popularity of their position. He argues that because they believe in freedom, and polls show the majority of Americans believe in freedom, than they all must be tea partiers, and that would make the tea party centrist. Even if that wasn't retarded, it wouldn't matter. He defined extreme as being on the edge of the political spectrum, which they are, regardless of their popularity.
I think you know exactly what I'm talking about and you're playing dumb. That's cool, it really just a matter of semantics and doesn't matter at all.