It digs into how we even section up and divide and measure brains in the first place -- this is suspect on many levels, for example, she cites that one paper that was huge in the 1980s uses 1800s openly-racist phrenology-based diagrams to section up the different areas of the brain. None of this is taking into account neuronal density, or any other such metrics.
Hell, there's outright a feedback loop where (for an example of one such feedback loop) what hormones your body is producing/you are taking, has huge impacts on how the brain functions and signals at a fundamental level -- changing hormones physically alters the brain, and there is some evidence in the literature that trans people (for example) have the physical characteristics of the 'opposite sex's' brain. Fausto-Sterling cites rat behavioural cases from the 90s where a single application of testosterone changed the rat's behaviour enough that the testosterone levels went up permanently, because the behaviour caused the rat's body to start doing that.
None of this is as easy or as clear cut as any article on the subject can describe, or even many graduate student books) describes and it is blisteringly difficult, and that isn't even taking into account the meta-science at work here where it is impossible to figure out nature versus nurture, and how much our own society changes our behaviour and changes our brains.
Related information is, there's a good twitter thread I replicated on imgur, that goes over in-depth about how the female/male scale does not accurately describe modern bio and neurochemistry (i.e. it has to be split up into many, many more discrete categories, more like a bimodal distribution) and actually is wildly inappropriate for certain drug interactions as it can cause incorrect predictions: https://imgur.com/a/qmIiULb