That is the point: "Plenty of 13/13 Windwalkers say [...]." Many people aren't at 13/13. In fact, a minority of the raiding population is. That does in no way diminish what they say, but their experiences can very much differ to what the average raider experiences.
And there, we differ - in the interpretation of the differences of the overall data / experiences compared to what many top WWs say.
Your theory (correct me if I'm wrong here, please) is, that many WWs are badly informed, also through many people spreading misinformation, and thus the average WW does fewer dps than an average WL (just one of the strong classes taken here as an example).
My theory is that it's mostly a scaling / RoRo issue. As an example: While the Warlock also has a good scaling and especially demo skyrockets with the castertrinket from Lei Shen, it isn't as extreme as the differences for a WW. And before you've got the Lei Shen trinket, no good WL would advise you to play demo, except on burst aoe fights.
Both interpretations lead to a very different outcome when it comes to class balancing.
[e] Though I have to say, the logical outcome of yours (something along the lines of "We have to get better, we have to be better informed") is preferrable than that of mine ("Blizz will fix it" - which actually it does in 5.4).