When I say carbon (or woodcarbon) in this case, I'm talking of the construction as a whole relative to the wood or technora construction. I agree the fiber per se is not interesting. But there is a difference in both feel and weight in between the respective constructions. Throw in the proto hollow boards that were experimented with a few years ago you have something different again (and here the you can really start to talk about flex characteristics). Interestingly, when you look at the pros that use custom wave boards, they might even have their personal favourite construction. Some (most) like a bit of give but some (like Polakow) like a stiff double vacuumed all carbon construction.
As for the physics: 20% is maybe an overestimation, but if you look at the EVOs wood is somewhere around 12-15% heavier than woodcarbon. And if we're talking percentages and assume the weigh difference is evenly distributed, the increase in inertia will in fact be by the same percentage. And in real life more of the weight loss is in the (more important) front part of the board so you will get a bigger effect than this.
And the whole point (which I tried to make with bikes earlier) is that you cant add all weights together and expect that to be the end of the story. Again, in a cutback, the board is doing a super quick rotation while the rider moves only very little and much slower. When doing such a move, you will notice more (15%) weight. But is it important in the big picture? Is it worth the extra money? Maybe for some. Maybe for some more if you factor in the psychological feel good associated with a light board. Maybe not for most. That's why there is still the tried and true wood construction.
Geo: Yes, that is a better formulation of what I meant. The board is kind of suspending the rider form the chop, effectively meaning you push less energy into the chop. Kind of similar to MX or MTB, in a way. A interesting question if you choose to believe in the proto iS tests (wood as fast as woodcarbon) is WHY this is so? You refer to the riders being super strong and sailing super powered up. But maybe some of the explanation also comes from the shapes. I'm not a slalom expert, but what strikes when riding for example the iS101 is that it hits chop in a surprisingly smooth way, maybe because of the rather flat scoop line and the rest of the nose shape. I very seldom get that feeling of smacking into something on the iSonics, they seem to fly over most and take the rest of the hits very gracefully. Maybe this is what makes board weight a less important factor?