I found that whole section (section 6) in the report quite odd.
My understanding is that the reason that there are male traits (e.g. aggression, and later libido) which are shown in stereotypical ways, and by extension female traits, is because of a combination of biology (testosterone or lack of, during various developmentmal stages including childhood) and socialisation. With children, this can manifest itself in which toys children select.
This seems uncontroversial as it makes sense that there would be a biological as well as a socialogical origin behind sex-based stereotypes. However, it's impossible to separate out the impact of nature v nurture in any quantitative way.
Where it gets unhelpful is when Cass refers to these stereotypical male and female behaviour profiles as a "gender identity". Perhaps it's an attempt to try and make sense of the ridiculous list of sex-based stereotypes that are used in the DSM-5 to diagnose gender dysphoria? Either way, it would have been far better if she had explored this without the phrase "gender identity" being used.