I think people misunderstand about the advantages of understanding fundamentals. Yes, very few are inventing new tools, protocols or reimplementing known data structures or hash functions. But the point of mastering the fundamentals is to transform the way your brain thinks when it approaches all sorts of computing problems. It's a framework that allows you to deeply think about the correctness and speed of what you are trying to build, backed by tens of years of research.

Unfortunately the most optimal thing to get your career started is not deep study of fundamentals but "grinding leetcode".

Fluency designing algorithms is a significant part of fundamentals though, if you can't do leetcode problems then you aren't fluent with algorithms. Practicing them doesn't necessarily make you fluent, but being fluent means you can do them.

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.

That depends on what kind of a career you are working towards.

Of course fundamentals are invaluable. But a stable career is equally if not more important to most people, I think.

I don't think Carmack has ever been "hired" for a job since he was a teenager.

So you're right, he's talking about "this makes you desirable in my eyes", not "practical employability through the lens of HR".