Yes, and no. The scene nodes don't render themselves but tell the renderer what to do, so far, so "yes". "No", because the renderer isn't the only thing that does OpenGL calls. The light manager for example makes them too, which granted, you can think of an extension to the renderer itself. But the texture classes, the shader class, the shader cache... All these classes make the OpenGL calls themselves, mainly because the renderer does exactly what its name suggests: Rendering.