Deep down in the report it is revealed that that 5.2% consists of a third of the class improving by 10% or more... and the rest remaining essentially flat. So it appears to me that it's particularly effective for a subset of learners.

Also note: the teacher doesn't have to really lead the class. The report says that the teacher spent most of their time 1 on 1 helping students who were stuck. Teachers may see value in simply having to do less work to achieve the same (or slightly better) result.

Also, this class was only a couple months. There is a question whether the small improvement would have continued and would have become wider when applied over a whole school year.