Ha - I’m this guy at work except I’m a couple of times more productive than everyone else because I know the codebases back to front and our team is several times more productive than other teams because the code is neat and tidy after continuous refactoring for years. We added 2 engineers to the team and they were both shipping features after a week of training because the code is neat and tidy. Having close to 100% test coverage makes refactoring easier, although sometimes new bugs do happen. ;)
Refactoring always requires a deeeep muscle-memory of the codebase, getting to that point at a job is a great feeling. Sometimes (always?) making things pretty pays off, too! :)
It depends on how the code is structured. Our apps use a micro kernel / plugin architecture of loosely coupled modules so most of the time it is sufficient to know only a few modules to be able to refactor. Fundamental changes require a lot more knowledge - this is true - but they are not common.
In my experience so far it is the structuring for maintainability that really shows the skill of the programmer - writing the logic itself is the easy part.
