They are indeed energy absorbers and I have personal experience in how they work. I was rear ended sitting at a red light. The only damage was the paint peeled off the bumper fillers. The bumper went in a few inches and back out. The car that hit me was towed. I think if I had had fiberglass fillers, which I do now, not only would the fillers be destroyed but also I'd have body damage.
The absorbers did what they were designed to do.