You are adding nuance to the underlying concept and failing to see how the wielders of the concept don't have that nuance.

Take BMI; first, i've seen arguments against BMI using the "there is no baseline normal" argument just like the original statement you quoted. Second, i've seen arguments that BMI as a concept is just invalid and rationales / facts that lend credence to the concept of BMI are somehow invalid. Finally, there's the inevitable ad hominem: it must be bigots who use the phrase BMI.

I don't think I'm adding any nuance, I'm explaining the context in which the quoted sentence was originally written. The nuance was already there, I just pointed it out, because it got lost when people selectively quoted that one sentence.

I agree that there are people who take any idea to its absurd extreme. I do not think the author of that article is one of those people.