The short answer is: their average IQ is 70.

I disagree, my current best estimate is the low 80s. The main reasons for this is various factors like parasites lowering IQ and lingering iodine and micro nutrient deficiencies have been empirically demonstrated to have measurable impacts on cognition and these factors are a bigger problem in Africa than elsewhere. Another reason is the analysis of other authors who tried to disprove his claims by using other tricks to try to infer g and the equivalent IQ (but could only rig the IQs up to the high 80s).