If I understand correctly from the various articles on this topic, there is a theory (with some evidence) that the lack of difficult chewing in childhood while the jaw is developing, leads to smaller mouths and tooth or breathing issues. Nestor's book does not make the claim that chewing tough foods as an adult makes a difference. What he does refer to extensively is a doctor that has an expander device to enlarge the jaw (which does work in adulthood).
He actually does make this claim very specifically. In his conclusions chapter, one of the main headings is "Chew", where he states: "the bones in the human face don't stop growing in our 20s, unlike other bones in the body." And then goes on to recommend eating "foods that require an hour or two a day of hard chewing".
I seem to recall that earlier in the book he describes some research that documented this effect, but I don't have a reference.
