Those mediums were probably chosen as they're all readily available and are slightly different than each other in terms of material stiffness and skin strength. Do you think that a chisel grind would somehow perform better in some other medium than a v-grind?
Keep in mind that, in general, almost everything you cut is an elastic material, and those will show results that align with the test results here.
I haven't owned a Douk Douk, but I bet it performs roughly in line with any other blade that's got a bevel of roughly 3/4" height and a blade stock of .11".
Carrots, potatoes and beets all are the same to a knife...hard materials that fracture with thick blades. Why do three tests with essentially the same media if not to skew results to a predetermined conclusion? Their test is designed to favor thin knives with v-grinds. In science its called "expectation bias" in experimental design and its a form of fraud or at least lazy thinking or just poor scientific technique
I know from experience that chisel grind knives are not the first choice for those types of things. Per Japanese chefs, chisel grinds are the best knife out of the drawer for some kinds of material. Also my electric paper cutter at work is a chisel grind as is the old tobacco plug cutter at the antique store.
The Douk Douk is a better slicer than the Mercator Cat that has a similar blade, but a different, standard v-grind and secondary bevel.