I think Carmack is absolutely atypical. What he says could absolutely be true for him, but that doesn't mean it's going to be true for everybody. It might only be true for a specific group of outliers.

I can totally imagine that someone who's sufficiently intrinsically motivated, and has enough mental capacity, willpower or whatever to put in long hours, longer hours might very well produce a linear increase in productivity. But most people are going to be exhausted at some point. Or demotivated. And enough of that is going to bring productivity down.

So it's the kind of thing that might very well work for you, but you can't demand the same thing from others.