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.