Well, yeah, actually I think gc views are kind of extreme when you get down to it, not many people actually believe that, they tend to be more moderate. But there was no shortage of fairly extreme feminist views in the 60s and 70s. Social constructivism was very big among academics and activists.
Even today, look at any thread about childcare, there are always people arguing that there is zero innate difference between women and men when it comes to interest in or suitability for childcare, or even that the hormonal differences aren't really relevant. Because they want to say that in order to have a non-sexist society, we need to completely balance the tendency for men and women to choose work vs childcare roles. And that's in the area that is probably more directly affected by biology than pretty much anything else we do.
That viewpoint grew out of the period when JM's research was seen as solid.
But there are two elements to the question of what this would mean, which it seems like people are really conflating, and they seem to be accusing Malaga of saying what I think it's pretty clear she isn't.
One is the "scientific" question, which is about things like nature or nurture, or what is socially constructed or not. Things that we can say are, theoretically, objectively true or not. They may be difficult to get at - we obviously have not solved the nature/nurture question. But whatever we think is the answer, how people function is a real thing.
There's also the question of what we should do about these things, which is a social moral issue, not a scientific one.
The gc view is that differerntiated behaviours and self-image as feminine or masculine is not innate to individuals. It is a set of stereotypes. Which JM's research seemed to confirm. It was hugely influential in the widespread adoption in society of the concept that there are not meaningful mental or innate psychological differences between men and women.
None of this means that people necessarily approved of his research methods (though mostly they were kept pretty in the dark.) Nor does it mean that they thought that he was right to think it was a good idea to impose socialization on anyone.
Most people seemed at the time to think that to some extent it was just inevitable that socialization would happen, though we could perhaps moderate it, but others felt this showed it was possible to just eliminate gendered socialization from society altogether if we wanted to. Which was very much what many feminists wanted to do.