The point is that you're not supposed to see it until you are done with the article, and start mousing around. Just the fact that you couldn't see it means that it did its job.

Now whether that's a good idea is another debate - it's a design decision to "hide" the sidebar.

But do this - compare the page with and without the sidebar - you'll notice that you focus on the actual article much better without the visual clutter of the sidebar in the way.