While I agree that Wout is better at sustained efforts and is probably also the slightly better runner the difference when both are in top form and having a great day is minimal. It should also be noted that Wout and Mathieu have different preferences in specific cycling disciplines and train accordingly. I’m certain that Mathieu has spent significantly less time in the windtunnel perfecting his position for time trialing (as well as climbing). I think Mathieu has proven that even with less training in that regard he could get great results if he did focus more on say time trialing (sustained efforts).Mvdp is the better crosser, having more explosivity and technique. So mvdp is almost always having an advantage. But as you say yourself, Wva has more (diesel)power, so the muddier / more running, the less of a disadvantage Wva has. That’s not a myth, as the results say so.
Furthermore, Mathieu has beaten Wout on mud courses plenty of times as well. I feel Dendermonde has propelled this “difference” to stupid heights.
I’d love to see a proper list of all the muddy/sandy courses they both have contested to see what the results indicate.