True, they save steel and weight with a hollow bar because the torsional rigidity is derived mostly from the resistance to twisting of the steel at the outer part of the radius rather than the steel they 'remove' from the core.
How can you tell whether your bar is hollow or solid for sure?
Try a heavier bar sometime on your car. Very satisfying.
Why is the math more complicated than computing the rigidity for a solid bar and then subtracting the rigidity of a solid bar with the diameter of the hollow space within?