It’s true that factoring is something you can spend an unbounded amount of effort on, but it’s too unwieldy to always say factored “so well that there is no point working more on it because of diminishing returns”. So when you see “correctly”, read that as “so well that there is no point working more on it because of diminishing returns”.

Arguably that is what correct factoring is anyway, because it’s simply not correct in any way to fiddle with a piece of software without generating any real value for either the business objectives or the developers that have to work with it down the road.