Numerous instances of feedback from AOs during the recruiting process proffered the view that IB Diploma is the hardest thing a kid can do in HS. Whether that’s true or not is I’m sure debatable (especially here), and I don’t care to debate it; but the point is that several AOs think it is and they influence admissions outcomes. So it seems those kids get a lot of rigor credit.
There can be more than one independent variable that influences something.