Because they explain the causal link. See the paragraph beginning with, "Most dietary carbohydrates relevant
for human nutrition contain the monosaccharide
D-glucose".
Countless studies show that consuming refined sugars alone has very different effects on the body than consuming complex carbohydrates along with the fiber found in natural foods. This is like saying that all protein contains glutamate so there's no difference between eating a steak or a tablespoon full of refined MSG.
I think their claim is that most carbs are mostly made out of D-glucose. And that excessive amounts of it cause your insulin based signalling cascade to go wrong.
I'm sure that encapsulating your carbs in a nice hard-to-unwrap cellulose structure slows the release of this D-glucose stuff and gives your body's control loop more chance to cope. But at this point we're arguing about the details of the causal link.