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I agree, it doesn't work for extremely muscular athletes but for the vast majority of people it works very well because the number of extremely muscular athletes in any country is small. So in this case it still serves its purpose very well.

A lot of body builders (even clean ones) are not healthy at all. 270 pounds is 270 pounds, whether it's fat or muscle. And 270 pounds of muscle is harder on the heart.

Many professional athletes shrink down to a healthy size after retirement because being so big is just bad for you.

That's all true. But none of that stops it from being significantly positively correlated with incidence of cancer.

What about when you control for bodyfat percentage? Genuinely curious. I've heard that waist circumference is associated with lots of negative outcomes even when you control for physical fitness, but I don't actually know this is true.