The notion that “IF YOU HAVE LOTS OF GOLD YOU PROLLY BOUGHT IT” is completely counter-intuitive anyway.
It’s more likely that players who blaze their way to Level 40 (or 60) are the ones buying gold, because they don’t have any. That’s the claim being made here: players who just quest/grind to level cap have less money because just completing quests and vendoring drops doesn’t generate money.
So when those players are broke and have a big purchase to make (fat mount, Dual Spec, epiccoolguy mount), they’re more likely to resort to RMT than picking themselves up by their digital bootstraps and working for their gold.
high time preference (in game) versus low time preference.