And he's not got to that question yet: "So this week we’re going to look at marriage patterns, particularly the question of age at first marriage. Then next week, we’re going to turn to the implications those patterns have for child-bearing and child-rearing"
But I think you're missing the other aspect of the population pyramid, extremely high child and maternal mortality. It's more that everyone, especially women, was a conscript in the War on Death. And even then people are generally having fewer children than humanly possible, because they're also starvation-constrained. From the article:
"We’ll be coming back to some of these points when we talk about fertility next week, but the mortality rate for pre-modern societies is very high, thus necessitating a lot of births, but it is not so high that societies need to approach a maximum ‘natural fertility’ (the birth rate using absolutely no means of birth control) to hit replacement and slow growth. But these are also peasant households with significantly constrained resources. So the question becomes how to restrain fertility to a high, but not maximum level."