I have no problem with the idea of down-weighting the entire national cohort to correct for this. The problem is that they actually weighted the results on a per-school basis.
That would be an improvement if your luck of the draw was national rather than biased in favour of private schools, but it's still a pretty awful outcome.
There may not have been any solution available that would be 100% fair to both this year and other year cohorts, but at the very least they should avoid making any student feel actively screwed over.