Lyman Stone, who is screenshotted in this post, attempts to address this [1]. His conclusion is that migration is not sufficient to explain the effect, but I have not looked in detail. In the past I have found his explanations compelling but I can't vouch for this one.
I believe that means you're in agreement with the author. The point of the tweet is to counter a claim that low fertility in cities is caused by high prices; if large families leave cities for extra space then they're not doing so because of price.
> I have written frequently about the relationship between high housing density* and low fertility. The most common counter argument is that the real problem is urban prices.
