Or how a label like 'human biological sex' is treated as if it is a true binary distinction that carves reality at the joints and exerts magical causal power over the characteristics of humans, when it is really a fuzzy dividing 'line' in the space of possible or actual humans, the validity of which can only be granted by how well it summarises the characteristics.

I don't see how sex doesn't carve reality at the joints. In the space of actually really-existing humans it's a pretty sharp boundary and summarizes a lot of characteristics extremely well. It might not do so well in the space of possible humans, but why does that matter? The process by which possible humans become instantiated isn't manna from heaven - it has a causal structure that depends on the existence of sex.