I read the book years ago and given the whole area of 'cultural' issues about food/weight management in endurance running am not surprised that the ideas on food limitation and 'ideal' weights are given little publicity today, in the UK at least. I'd guess very few coaches would feel comfortable advocating his methods in quite the 'full on' way his book sets out. He may have published the book at just about the time that anorexia was getting any publicity/awareness outside the medical world - early 1970s tho apols if I have dates wrong. The book I recall had little content to suggest any link between this highly austere dietary management and the psychological mess that can ensue(yes I've simplified hugely there)
As for these lightweight Africans, from published data on elite enduros from wherever, I'm really not convinced that their best are signifcantly lighter than the best of Japan, Spain, USA, GB or wherever. And even elite Africans vary on the % below weight-for-height scales - Tergat vs Khannouchi as one obvious example.