If it's not chromosomes
It isn’t ‘just’ chromosomes though. Because they are people with differences in sex development with chromosomes that are different to xx.
If you read more from the scientists, being female is described as having a body that is designed to have the potential to produce large gametes, whether it does produce them or not.
It also encompasses female reproductive systems and genitals and hormones etc.
Hence why activists jump on the sex is a spectrum wagon. However, sex is binary because to reproduce, it requires a large gamete (female) and a small one (male). No other combinations are possible for reproduction and no other sexes have been named.
That is often what they mean by women are not just XX and out of date. On one hand they are right, and anyone with a DSD is either male or female but chromosomes will vary.
On the other hand, they want sex to be defined as a spectrum (the activists) so they can destabilize the definitions of female / male so they can say people are ‘female’ because they have artificial hormones (ie cross sex hormones) and surgery to construct genitals of the opposite sex. While conveniently missing the point that their cells are coded forever to be the sex they were born. If they stopped the medication their bodies would try to produce their own hormones if the organs were still intact and viable.
Activists politicize other people’s medical conditions to further their need to destabilize language and science. The ‘spectrum’ they call the sex spectrum is actually a spectrum of body differences within each sex classification. This has also always been true.
Sex itself is not a spectrum and if your body is male, it will always be so until you can magically rebuild yourself from cell level up.