I agree with this. However, like I've mentioned before, these comparisons are silly. There are plenty of examples where the one beats the other. The comparisons become especially problematic when we start comparing their wins vs each other on each's favorite terrain or the one that suits the talent of the one over the other. Comparing Wout's wins on longer climbs or tt'ing vs Mathieu is just as silly as comparing them on explosive finishes.
The argument that goes Mathieu's way is that there's no doubt about a trade-off when combining different disciplines vs being focussed on. The difference in teams and goals for said team and how much time/capital/energy is allocated to achieve such goals is also evident.
In any case, Mathieu's choices aren't Wout's fault so he can only focus on his goals and do everything he can to achieve them, and he has done so quite successfully. So kudos to him.
If I'm totally honest I think Mathieu could have had a much better palmares on the road by now if he had put XCO aside. His big goal as far as XCO goes turned into a nightmare and I just can't help but think that it's all been for nothing. He can't properly participate in the xco cups as it stands in the way of his road schedule and has specifically adjusted his xco/road schedule in order to qualify for the xco olympics. Let's see if he changed his mind after that fiasco.
I don't even want to get drawn into this "x is better, no y is better" discussion, they both have an incredible strength and maybe one is actually more talented overall than the other one, but I don't really care about that because like you say it's hard to actually compare them, and they both are monsters. I just think belittling one of them by stating that the other is far superior overall like some fans do, is, as it stands, not adequate or fair.
How both of them manage to combine the disciplines - even if van der Poel does quite a bit more of that - is astounding.
Very probably van der Poel could have a better road palmares if he focused on it, but on the other hand I wonder if his status as an active cycling legend could be bigger if he did - probably not.
Doing everything for one goal and then see it all go wrong within a moment is probably quite tough, on the other hand I think it's not such a big thing after all. Think of all the athletes who are training and training, over years, for a moment in the Olympics, having to find the money to even be able to train and pay the rent, and then it's all for nothing because they are sick on the day or they make a small mistake in the moment... it's a decision you make, it's the reason why something like an Olympic medal is such a thing for many people. When everything goes well it can look easy, but in many sports it means giving your all for a gamble...
What I want to say, compared to that van der Poel is pretty well off. He's earning very, very good money and just because he had to give up certain goals on the road this year and probably also the years before, he already has a pretty good palmares and his stunt in yellow in the Tour this year for instance, something that looks like a by-product for him, is something most cyclists would consider an incredible dream.
He's also still quite young and if he should decide to focus more on the road, he still has a lot of time to do everything he wants there. I just don't think he will in the next years; I don't see him having the pleasure on the road that he seems to have if he combines the disciplines - but then of course I really don't follow him enough to have a good clue.