Edit: Since we are talking about Carmack here you probably know he is among other things famous for designing custom algorithms for the games he made. So when he talks about deep understanding it includes deep understanding of algorithms, how they work and how to make new ones. If you got that down well then leetcode isn't hard at all.
So you're right, he's talking about "this makes you desirable in my eyes", not "practical employability through the lens of HR".
* If you've mastered git commands, you haven't followed Carmack's advice.
* If you've used git as a domain to understand key-value stores, Merkle trees, hash functions, and network theory, you have followed Carmack's advice.
Or another example:
* If you've mastered using a gaming library to move characters around and memorized APIs, you haven't followed Carmack's advice
* If you've mastered a game library to understand 3d transformation matrices, GPGU, and simulations of physics, you have.
A tool can point you to an aligned, coherent set of fundamentals which also work together to make a package useful for employment.