The claim that aluminum frames dent more easily is just another generalization that doesn't always hold up in specific cases.

All things being equal the tubing of an aluminum frame will have more than twice the thickness than a steel frame. However things are never equal. In order to improve stiffness, a typical aluminum frame will have larger diameter tubes spreading the same volume of material thinner.

So modern aluminum frames have large diameter fairly thin walled down tubes, but a modern high end steel tube, ie. 531 or better will be even thinner. So a high end aluminum frame may dent easier than a low end steel, or aluminum, frame. But a high end steel frame will dent just as easily if not more so.

So, don't crash and let your handlebar swing into your frame, or drop your bike against a parking meter.

Steel, aluminum, Ti or carbon fiber, a high end bike is like a good sports car, simply not built to handle the abuse of a F-150 pickup. Treat it accordingly.