In a similar way, one might argue that "races" don't really have any objective existence, and yet still acknowledge that e.g. African-Americans are less likely to go to college than Asian Americans. Such stats aren't invalidated by the lack of any objective biological definition of "race", or by the existence of some people with mixed African/Asian heritage, etc. etc.
It fairly obviously is not. That sex traits are multidimensional and that there are more than two ways that that they present in combination is...not really a matte about which there can be serious, fact-based debate.
> and that the existence of intersex people somehow disproves the idea of male/female sexes.
It (and a number of other phenomena that aren't typically labelled intersex, including, but not limited to, the way certain physical sex-linked traits other than external genitalia in transexuals are, on average, more typical of the sex corresponding to their gender identity than to sex identified by their at-birh genitalia or combination of sex chromosomes) obviously disproves the idea of exclusive stereotypical male and female sexes.
But none of that is relevant to the upthread issue, which was the supposed claim that males and females are mentally identical. That is a rather distinct claim from “sex is more complex than a male/female binary distinction”.