Actually that was referring to how little deflection happens between zero and torqued--you can feel the difference in material with different rails.
Anyway, aluminum rails are generally most likely to be damaged by rings, usually the cheaper ones without a quality hard coating. And since steel rings are harder, they can be more likely to mark one up all else being equal. Of course all is rarely equal so it isn't that simple. The design of the rings is most important.
Rings like mine could be made from Ti or steel and it would have zero affect--they still wouldn't damage a soft rail. A rounded surface on the clamp instead of a sharp edge, a ton of bearing surface on the lugs to spread out the recoil force, etc, keeps them from damaging the rail and making them from steel wouldn't change that.
On the other hand, especially with heavy recoiling rifles, many cheaper weaver-style steel and aluminum rings use a round crossbolt that can dent the recoil groove on the base. A sharp edge on the clamp can also leave a mark.
Even some high quality stuff can do it with enough recoil--I had a pair of badgers put dents on a (really soft) base on my 300 RUM with a heavy scope. There just wasn't enough bearing area on the crossbolts to spread the load out enough. This is one of the reasons I designed my rings the way I did.
In short, with a steel or Ti base you really don't have to worry about it. I also like those for general durability--I like to permanently bed bases to the rifle so using one less easily dented or scratched from abuse makes sense. They also add a bit more stiffness to the action.
Ti is expensive though and steel is heavy, so aluminum is a good choice for many who are saving ounces. Just keep in mind you need to pay a bit more attention to your ring choice--design more importantly than material--if you do use one, especially on a really heavy kicker.
All that above is for picatinny bases. For weaver bases, you've got a round slot so rings like mine don't work. You're kind of screwed in trying to spread that load out. For a weaver base on a heavy kicker, I'd always go steel regardless of rings to be used.