Maybe because "it takes a village to raise a child" and we don't have villages (= tribes) any longer, so only fathers are actually around enough to count. It might not be true, but it's at least plausible. It's worth checking. The Dutch study says: not true in the Netherlands, not as compared to same-sex couples. Great! What about even less village-like places, like, say, New York City or Dallas?

As to who could be confused--well apparently anyone could be as you admit given that you say, "the past beliefs about this were driven by cultural assumptions". There were papers, you know, with statistics, whose results were interpreted in light of those cultural assumptions. I don't think it's unreasonable here (or ever, really) to not fully trust expert opinion--even Hrdy, who I agree is fantastic!--and say, "Show me the study."

(Heck, I even generally look up a bunch of Haidt's references, and he's about as careful as anyone is to get them right and draw well-founded conclusions and warn you when he's extrapolating.)

Also boys are not doing as "just fine" as they used to be. Richard Reeves is one of the more prominent authors explaining in what way, e.g. https://www.deseret.com/magazine/2022/11/9/23404485/boys-men-mental-health-masculinity-richard-reeves. I don't really agree with his solutions, and there are no references there, but at least he does a decent job of introducing many of the problems in that interview.

Anyway, you certainly aren't obligated to keep discussing the matter. I think I've fixed my original comment to convey a fair degree of critique of missing sources. Please let me know if it is still lacking. I've left notes in the text so that your first reply still makes sense.