His expertise over a very wide array of objects is also extremely impressive.

I know spectroscopy very well, and yet hearing him talk about light-matter interaction was still very educative for me at the time and I was impressed by the depth and sheer knowledge he had on the matter, learned many things I didn't even know existed from him.

Same applies to many other topics related to physics, engineering and mechanics, not just computer science and programming.

Whoever says that you need a degree to get depth in several topics has never knew people like Carmack who simply study and try to understand.