Probably pretty valid.
Keep in mind, too, that in some H-G societies when there were multiple births only one of the babies was allowed to live. The H-G lifestyle is not very compatible with multiple infants at the same time.
That is only true in the modern world. Go back 100 years, or 120 and women generally didn’t start menstruating until their middle or even late teens. In the past people weren’t as well nourished and generally didn’t reach sexual maturity as quickly as they do today. I don’t know if menopause tended to come sooner or later, it’s possible we simply don’t know. There may have been times in history when most women didn’t live long enough to experience menopause.
As for when natural fertility peters out… I believe the record for natural conception without modern medical technologies is around 56… but that’s as much an outlier as the young girl who gave birth at the age of 6.
I’m guessing that in the past, meaning pre-modern medical care and with less certain food supply, most women only had about 20 years of fertility, with the occasional surprise baby in the 40’s. To have more than the H-G 4-8 kids in a lifetime you need agricultural food production. 10 kids in a family wasn’t that unusual post agriculture (my mother had 9 siblings, for example, what was considered unusual was that they all lived to adulthood) but getting beyond that was remarkable. I’ve heard of families with 15 kids all from one mother but those families were considered quite unusual, and probably had at least one pair of twins.