I think people are focusing too much on hollow vs non-hollow. The mopar hollow bar was designed with the amount of flex that it has. If they (or any manufacturer) wanted to make a hollow bar have less flex, they can do that through various methods (not necessary to cover those methods here). On the same side of the coin, a solid bar can have the same flex as a hollow depending on how it's designed. The key here between the hellwig and the mopar are the leverage arms.
You can easily see the differences in the leverage arms a couple posts above in IvoryHemi's post. While it's not exact, putting the end links in the outer hole provides a very similar lb/in rating between the hellwig and mopar. As you move the end links inward on the hellwig, you shorten the leverage arm and increase the effective lb/in rating, making resistance to sway greater. It's really as simple as that.
Of course, the hellwig could be a combination of solid + shorter leverage arms, but they feel so close when the hellwig is in the outer hole that I suspect the rating is very similar. I'd like to see the lb/in rating of both when the endlink is in the outer hole of the hellwig. I'm guessing someone will need to contact hellwig for that info.