> How is this different from when you join a company and learn your corner of the codebase?

Because most AI generated code is incredibly verbose and noisy. This can be true for any codebase, but it is especially egregious in AI codebases. It is hard to prevent that by just saying "be concise" because knowing exactly where to be concise is the measure of true skill/intelligence.

Every time I have success with AI coding, it is because whatever I need done can work as a pure function, where the probability I have to return to it is very very low.

Making sense of a codebase is like putting together a puzzle. A purely AI generated codebase is like trying to put it together without a picture on the pieces.