However, now consider for example aggressiveness. In pretty much all societies, males are considered to be more violent and aggressive than females. ... But the claim that gender as a whole is a pure social construct is empirically wrong.

Not all attributes of gender are transient; not all of the things commonly attributed to one gender or another would be totally gone in a genderless (non-transhumanist) society. But that doesn't mean that the category of gender cleaves thingspace at the natural edges.

Aside from looking at other cultures, there are other methods. For example, one can look at children who were genetically male but had surgery at a very young age that made them anatomically female. Even when they are raised as "female" they [frequently develop habits, attitudes, and play interests] that are considered to be masculine.

This isn't a good experiment unless the people involved have no knowledge of the child's birth sex. Also, as far as I'm aware, there's about one prominent case study describing this, and as such I think it's misleading that you use the word "frequently". The one time I know of this happening, you're right -- the person eventually transitioned back to masculinity. But that's not much to update on.