He admits that he works long hours to get that productivity though[0]. He's probably 10x better than many who would work the same hours as him, but I'd be willing to bet if he limited himself to 40 hours a week he wouldn't still be 10x better. (Not trying to make any judgements either way, just an observation & prediction.)

0 - https://twitter.com/ID_AA_Carmack/status/684475417726595072