in which the Flynn IQ increase was above 1SD in many countries. If the number of young potential von Neumanns increased dramatically during this period, as it should have according to the simple normal distribution model, then the teachers should have been struck by how more and more kids find the standard math programs insultingly easy

Well, as discussed above, there are many interpretations of the Flynn effect, and it's not clear that the IQ increase actually corresponds to a gain in intelligence. From what Flynn has written, it seems most likely to be a measurement problem of sorts, in which case the number of "potential Von Neumanns" would not increase.