Why? Carmack is an extreme outlier and it's likely that even among the people he knows that he would label as qualified for the position nearly all have at least a master's degree and the other requirements.
I would bet he doesn't or at least thinks the minimum requirements are overkill, otherwise why would he make a remark like he did?
The point is that when a legend like Carmack doesn't make the MINIMUM REQUIREMENTS maybe there is something wrong with the requirements.
The top names in any given field tend to know each other (it's a pretty small group and they network) and most of them got their degrees and other qualifications the traditional way. As for why he made the remark, IMO it's a self-deprecating joke about his being an outlier.