That explains how it works, and it works how they were asked to make it work (ie no grade inflation), but doesn’t really explain the problems which caused the Govt U Turn.
Ignoring that the U Turn was undoubtedly impacted by public perception (because the public are, famously, also voters) the main problem was that the system is inherently discriminatory in the first place and therefore when you try to write an algorithm to replace what the system usually ends up with as an outcome you’ll end up writing a discriminatory algorithm.
This article is a good one, and has useful links.
https://www.technologyreview.com/2020/08/20/1007502/uk-exam-algorithm-cant-fix-broken-system/
One quick anecdote to explain why is a friends child who was assessed as being level 7 (don’t ask me to explain levels though...they are way beyond my understanding!) took mocks and got level 8.(a good result apparently). However more children at that school did well in mocks, which are independently assessed, than usual and so the algorithm decided they were just above middle in their school, so gave a result which put them just above middle in previous years results...so they got given a 5 instead!