There is still some interesting stuff there, like the fact that they are only using 1 year of data for maths cohort progress as there is only 1 year of the linear A level.
I agree that it is impossible to design a statistical system which works every time, but I can't see how any system will work for schools other than the ones with consistently high results. I have said this before in various places, but at DS1's school they got 14% A in Maths A level last year, 3% the year before with similar sized cohorts >70. No idea how you interpret that, but then for Product Design they got 100% As one year (a cohort of 2) and nothing over a C in another year (cohort of 5). Not sure how well GCSEs would map onto a subject like Product Design as you do not have to have done the GCSE.
My main issue with the system they have developed is that you REALLY need to be able to appeal if the results look silly. There were a lot IB students who ended up with silly results - a parent from a school where all 17 people with medicine offers failed to get the grades posted on TSR. The system starts with a personal approach (teacher assessment) and ends with a statistical approach when it should be the other way round, the assessment of the individual should carry more weight than the algorithm. If there was a better system for schools to be able to appeal if the results are clearly wrong, I would have more confidence in the system.
From where we are now, I think that this year may have been more successful if they had turned the process round and done the statistical analysis for the schools and allocated a pot of grades to be distributed across subjects as fairly as possible, then maybe had a second round of statistical approach if the subject grade statistics were unacceptable. A top down approach where teachers who know the students made sure they got as close to the right grades as possible would surely have been better than what we have.
Maybe it will all be fine; the fact that teachers have ranked students will hopefully mean that students can not slip more than 1 grade in each subject unlike the IB.