He does run his suspension stiffer than anyone, however that might not be the reason, it's only speculation. It's as much proof as saying when he was not Gwinning, the reason for it was that he was running his suspension too stiff.
The issue with stiff suspension isn't just about the rider being able to hold on and fatigue issues, but traction is lost, likelihood of punctures and issues with your wheels comes into play. The dynamic geometry of the bike is also effected a great deal (for example, running your rear end really hard on steep tracks, will pitch you over the front)
Take Jared Graves as another example. Ive read and heard from those closer to the pros, that he runs his suspension significantly softer than what you'd imagine. Now, I agree that you can't compare DH to Enduro racing, but should be some indicator that there's no magic bullet and that everyone should run what works well for them. (which is why I always find it so strange when I meet riders who haven't played around with their bike set up at all, to find out what they like / dislike)