In a "well-documented, well-structured codebase with fast built-in feedback loops", a human programmer is really empowered to make changes fast. This is exactly what's needed for fast iteration, including in unfamiliar codebases.
When you are not introducing a new pattern in the code structure, it's mostly copy-paste and then edit.
But it's also extremely rare, so a pretty high bar to be able to benefit from tools like AI.