My argument is that no one can claim BMI or IQ is a scientific form of measurement.
Maybe they are crude. Maybe there are better alternatives. That doesn't make them unscientific.
Those who double down on IQ as being infallible have something to lose if it's proven to be hack. And history is not on your side here.
No it hasn't.
BMI doesn't work for three groups of people: it makes very tall or very athletic people think they're fatter than they are, and it makes very short people think they're thinner than they are. But, as GP says, that's less than 5% of the population.
History is very much on the side of IQ realism. Ten years ago people were denying that a general factor of intelligence existed at all, now it's down to nitpicking or argument from ignorance.