Quite strange… do you see that here on Meta as well?
This wasn’t on our radar yet. As we get a bit further along, we’ll probably enable it there by default, but right now, this is still experimental, so it’s a bit early to do that.
Adding elements of the sidebar to the styleguide may be worth doing as well.
We have in mind adding support for a query parameter that would allow you to enable it yourself though. This would also allow you to preview your site’s own existing theme with the sidebar prior to deploying it. But we haven’t gotten to that yet.