They're finding out what grind profiles different grinders have. This has nothing to do with taste within the confines of this test.
They're just dialing in the grinders pulling shots just so the grinders are set to comparable, real world settings. Of course, this has implications for taste but that's not what this research is primarily focussed on. The question here is not "which grinder produces the best tasting shot?" but simply: "which grinder delivers what kind of distribution?"
There are going to be correlations between those two questions but their research is focussed on question #2. The theory behind what that means for question #1 should really be discussed outside of the actual research they're doing but anyway: My understanding is that a grind that is all over the place is going to result in under- and overextraction at the same time with a fair amount of proper extraction in there. How large that share of proper grounds is (the large hump in the graph) determines how good a grinder is according to this hypothesis.
Put simply, say, the optimal extraction for your coffee A would happen at 18g of 200µm particles. That's the shot you like, that's how that coffee is supposed to taste, that's the godshot, the maximum potential of that bean. But: You could get the same extraction time and shot weight if you use, say, 14g of 130µm particles or 22g of 270µm particles but the best extraction happens with 18g of 200µm particles.
Now, of course no grinder is going to produce a perfect 18g of 200µm particles. That's NASA stuff right there. An almost perfect result would look similar to that Ditting curve with, say 80% of the grounds within the 170-230µm range. Great. If you have a Mazzer Mini, however, you're only going to get 40-50% in that range and then, say, 30-40% around 350-450µm and another 20% or so all over the place. That shot is still going to be good, because 40-50% of the coffee are properly extracted. The rest, however, will either be over- or underextracted and you'll probably taste that. It's always a mix between *both*, over- and underextraction. If overextraction dominates, you might grind more coarsely but that'll also move the 40-50% of "proper" grounds away from the optimal range, so you'll go back and you're never able to absolutely nail it. You'll never know the full potential of that bean, *if* uniform grind size distribution is indeed optimal - we don't know that yet! Some people might argue you need fines to seal a puck properly...
An early hypothesis, looking at their data, would be that burr grinders have two humps:
While konical grinders have one larger hump and then a fair mess of larger particles:
It seems that the Robur has a larger portion of "proper" grounds and then some larger particles that are going to be underextracted. It could be that that cup tastes better because there's more "proper" extraction happening and the larger grounds don't matter as much and just result in a thinner cup. It could also be that the optimal setting for the Major is between those humps as it appears to become smaller with seasoned burrs... Now, this is where taste testing would come in. But it'd be educated taste testing at this point, not just "I prefer this grinder, I don't know why, who cares?"
So, tl;dr: Kudos to them. If we try the same coffee from different grinders in the future and clearly prefer one, this data might put us in a position to make an educated guess as to why.