> Either you need to understand how your codebase works or you don’t.

It's an interesting point. We can also think about it perhaps as a non binary thing - you need X amount of understanding in a specific codebase to be effective. Even before LLMs in large codebases no one understood it all; but we at least mostly understood our own PRs and our own areas of expertise in the codebase.

I have a very good mental model in my head of each and every codebase I have ever worked on. Nothing to do with my PRs or even my team’s direct responsibilities. I could always direct a thought provoking idea to challenge the existing status quo and get a reasonable nuanced answer from code owners on slack or during a water-cooler talk. I build that mental model also by reading the code. This is my job, so I know when to tear down what or how to respond to a random product idea immediately on a meeting to asses complexity.

Before LLMs you either built good abstractions to make it possible to not understand large chunks of the code base or you flailed.

A lot of the time people flailed.

The one thing Ive never seen an LLM do well is shape clean, coherent abstractions. To be fair it's a rare human skill as well but in LLMs if they don't have a direct analog in their training data they flail.