I don't agree. Garnett was simply more talented. We heard constantly during Wallace's time in Portland that he and Duncan were "equally talented players" but Duncan cared and "wanted it" while Wallace didn't. The funny thing is that most of the reasoning to illustrate that Wallace didn't care enough was not playing in the post enough. What makes that funny is that Wallace's game would have been celebrated as ideal in today's game. A power forward who could stretch the floor and play great defense, and even get some points in the paint.
Duncan and Garnett were two of the most talented players in NBA history. Wallace was extremely talented, but a tier or two down. It doesn't take a "mindset and approach" theory to explain Wallace not being as good as Garnett, in my opinion. If Wallace didn't care and work, he wouldn't have been the great defender that he was.