What matters less is the mechanical knowledge of how to express the solution in code. The LLM generates code, not understanding.

I think it's the opposite -- if you have a good way to design your software (e.g., conceptual and modular), LLM will generate the understanding as well. Design does not only mean code architecture, it also means how you express the concepts in it to a user. If software isn't really understood by humans, I doubt LLMs will be able to generate working code for it anyway, so we get a design problem to solve.