Either we admit we'll never budget maintaining the comments metadata on top of the code, and embrace that risk for the cost reduction it provides, or we do document for real and maintain and review and fix regularly at a cost.
But saying we dont need to do it because it s already done is a quick stunt that everyone saying it knows is dishonest.
I've been programming in one form or another since I was 8 years old (around 38 years) and this was obviously incorrect to me back then, even though I wasn't sharing my work with anyone else, and hadn't heard this concept.
Years later I was astounded to move into a "modern" dev environment where people (developers, devops) parroted this line regularly. As you say it's nice in principle, but in reality relying on it proves to be impractical.
It's one of those things that sounds nice, but once you've moved beyond a certain level of complexity you realize how impractical it is. The fact is that "good code" is often in the eye of the beholder and not everyone has the same skill or vision, so they might as well write a comment about what something does and the intent behind it rather than leave others guessing.