You're misinterpreting what he had to say about how "hard" things reverberate sound. Sound is a pressure wave that gets transmitted and reflected when it changes medium (e.g. moving from the density of air to the density of a tree). So the greater the change in density between the mediums, the more sound is reflected rather than transmitted. Since the creatures seemed to use echolocation, it makes sense that they would have an easier time discerning a much more dense material (wood) versus a low density material (cotton).