That is not health advice, that is "how to be anorectic" advice. If you are at the weight with longest lifespan, you are somewhere at the top of "normal" bmi range and wont see ribs. You wont seen them in the middle of "normal" range and plenty of thin wont see them either.

Otherwise said, for quite a lot of people accomplishing this would mean underweight.

Underweight is less healthy the overweight.

It’s unnecessarily crude heuristic but it’s not totally wrong. It seems like you can see ribs in adult men up to around 25% body fat which is around the end of healthy range.