That's what they're doing. They're weighting someone's coursework by an estimate of how they'll do on the test, based on how well students at their school have done on the test in previous years relative to coursework in those years.

No, a student in the 90th (top 10th) percentile of coursework at a low performing school is the ones who are harmed

Of course, they are harmed by the fact that their school makes coursework too easy. The alternative, of course, is using unweighted coursework grades which harms the students at schools that make the coursework extra hard.

The problem is that the IBO doesn't actually know what a student's abilities are until they write the exam. They also don't know how hard the coursework is at their school, in an absolute sense. They only know how hard the coursework is relative to the exam, based on a comparison of previous years' course grades with exam grades. Schools that consistently make coursework too easy will inflate the grades of their students. If those students underperform on the exam, the IBO will know that those schools are practicing grade inflation and adjust accordingly. That's all there is to it.