I'd argue that a lot of his success was directly related to not knowing anything about SQL/RDBMS because they were not required by his specialty.
Instead of spending time learning SQL, which he'd never use while creating a game, he only learned and practiced the skills he used for games. It's incidental that a lot of those skills are fairly universal and low-level, which gives us the false impression that Carmack is a generalist.
Why not? He doesn't need to use it for his work, why would he spend time learning it vs learning something relevant for his field? Oh, I know, to pass a GYMAAAE interview, but that is a pretty bad reason.
There's a universe of programmers that don't work in the LAMP stack (including me) because it doesn't suit what they are trying to accomplish.
Instead of spending time learning SQL, which he'd never use while creating a game, he only learned and practiced the skills he used for games. It's incidental that a lot of those skills are fairly universal and low-level, which gives us the false impression that Carmack is a generalist.