The point is that if cancer and heart disease are disease of aging, but were rare in paleolithic times, then a life expectancy lower than the age at which they are expected to strike does not preclude the possibility that there were other factors than early death that reduced their incidence.

If a disproportionate number of people died before the age of 5, a life expectancy of 40 means that many lived beyond that age, and would be expected, all things being equal, to have contracted cancer and/or heart disease.