You state that there is nothing cultural about birth rates and then list several cultural factors influencing birth rates. And then you state that there is nothing we can reasonably do to increase birth rates, even though most of the factors you mentioned are not set in stone.
I don't think the person being replied to necessarily meant that, but cultural is often used in this context as a racist term, as in: "Africans will always have many kids, it's their culture", where before the more overtly racist "nature" might have been used.
The implication being that the cultures of (some) people are unchanging and fixed. I think you'd agree with a statement like: Cultures and family values do change all the time, due to a variety of factors like better medical care, and economic empowerment, in reasonably predictable ways.
So it's not unchanging cultural differences between Africa and Europe that drive the different fertility rates, but rather different socio-economic factors that drive a different family culture. Changing the socio-economic factors will change the culture surrounding families, just as much in Africa as it did in Europe, the Middle East, Asia, etc... at various times in the last century and a half.
My argument was that all modern, technologically advanced civilizations will face these exact issues, so there is no point in calling those factors "cultural", because cultural facts are meant to distinguish between cultures.
I'm sure an alien species or a computer simulation having a few of the same basic drives as humans do (sex is very pleasurable, contraception exists, raising educated children is extremely expensive and hard, state provides pensions in old age, people are free to have or not to have children, child mortality is low, work provides money, money satisfies needs, quality work is abundant only in high density cities and a coupe of others) will arrive at same fertility problem we have arriven at.
My second point was that there is nothing we can reasonably do without a major overhaul of human traits or shutting down the technological advanced world we live in.
I'm not even sure what you are trying to say.