I would not say that is not necessarily the defining piece that creates the smooth upper range, but it is certainly a component of the design
I have heard many design theories - internal veneering (Totem), minor internal bracing (Harbeth), built like pieces of rock (Reference 3a), inverted driver sets (Mission), to name a few - and without a competent design, quality materials and good build quality, the speakers fail to reproduce music properly. All of the above manufacturers make good to exceptional speakers. But it is just as easy to take expensive parts and make bad speakers - or any electronic device for that matter
We can go into a "What makes a speaker reproduce music properly" discussion...but a search will bring threads about this to light.
