I don’t think that is true even today.
https://en.wikipedia.org/wiki/68–95–99.7_rule: ”in a normal distribution […] 68.27%, 95.45% and 99.73% of the values lie within one, two and three standard deviations of the mean”
So, your claim is that only the bottom (and, possibly top) 2.5% of the distribution need to worry. That corresponds with IQs of below 70/above 130 (https://en.wikipedia.org/wiki/Intelligence_quotient)
Also (https://en.wikipedia.org/wiki/Intelligence_quotient#Job_perf...): ”The US military has minimum enlistment standards at about the IQ 85 level. There have been two experiments with lowering this to 80 but in both cases these men could not master soldiering well enough to justify their costs”.