This only really holds for older people, who have basically no ability to recover lost muscle mass after a serious illness which makes it difficult them to continue to exercise.

The correlation is unambigous. The causation is a different story, e.g. illnesses often cause weight loss.

Which segments of these populations have high BMI because of muscle mass instead of excess body fat? Which segments have low BMI due to childhood malnourishment or current or chronic illness? These are just some of the very obvious questions that have already been used to relegate your conveniently concise "fact" to "interesting, but not instructive".

> A few extra pounds when you are older helps you survive illness.

This doesn’t mean being slightly overweight from age 20 through 60 is an advantage.