Some of the original details - slight as they may be - indicate that we are talking here about putting off 'formal' instruction and that, in fact, students engaged in 'informal' mathematics activities in the earlier grades.
It seems that this 'putting off', so I have been told, still occurs in those certain private schools where students craft their own curriculum. Similar results, I am told, often follow.
In any case, the seen failure of teacher education would seem to be dwarfed by societal and parental opposition. There is no indication, by the way, that Benezet's teachers received any sort of training other than being told, in essence, 'hands-off.' Perhaps in today's parlance that would translate into no 'direct instruction.' However, when formal instruction was begun, it may well have been quite 'direct.'