It's a fine engine, just suggesting that some of the reason is that the games running on the engine don't necessarily stress the hardware as hard as others do.
All that said, John Carmack hasn't always been right about things. For instance, he was - for a long time - an opponent of using hardware 3D acceleration in games (like with Glide or Direct 3D) claiming that software rendering was the only way to do it right
He also was lead on the engine development for Rage which was widely panned for its texture issues due to trying to render with dynamic quality in order to keep framerate constant.
He has been hugely influential in getting us to where games are today, but no one is perfect
Wow so you've deviated all over the place again. ok. What does this have to do with more cores? Anyways the beauty of the Rage engine (idtech5) was cross platform. Microsoft Windows, OS X, PlayStation 3, PlayStation 4, Xbox 360, Xbox One