Just because Emma Watson's definition of woman is different from yours doesn't automatically mean she is wrong and you are right. Surely you can see it's more nuanced than that.
If transwomen are women, why can't women be transwomen?
The fact we can disguish a different is because...?
If we can not define sex we also can give legal protection to either women nor trans people because there is nothing to define them, apart from using gendered stereotypes on the way men and women are traditionally supposed to dress. Aka sexist views about masculine and feminine ideals.
Emma Watson can not be right in her opinion not because I'm being an arse about it or merely disagree with her, but because if you do not have sex as a definition the language of describing things and people evaporates. Medical knowledge and treatment appropriate for men and women disappears and degrades in quality. The ability and opportunity for females to compete in sport competitively vanishes. Discrimination in the workplace against women because they may or may not have children becomes invisible. Women lose political representation and they lose career opportunities at the highest levels because gender ratios are used instead of sex ratios. The gender pay gap can be fixed merely by having enough Self Iders. Now why wouldn't that be attractive to an unscrupulous employer? Here have an extra couple of £1000 to use the right pronouns so I don't have to employ too many women and worry about costly inconveniences like maternity cover ever again.
If women are not centred because of the barriers that sex brings them, then you can not be being feminist. If woman becomes a concept rather than a physicality, it waters down what opportunities, protections, specialist services and medical research women have that serve their sex based needs.
For example: How can you study the impact of the menopause on the body without recognising sex, and excluding male samples from your research which may complete screw up results? There are enough cases of transwomen claiming they have had periods and gone through menopause to realise that, no this suggestion isn't as nuts and out there as you might think. Sadly.
JKR said that we need to always see sex cos sex is an important part of how humans interact and whilst people might identify a certain way sex still remains important to them even if they don't realise it. This isn't controversial. This is an acknowledgement of how our natural state of being isn't something we can change because we feel entitled to or because we somehow feel its wrong. Denial about this, won't fix internal identity conflicts and issues. It merely surpresses them and places a burden on society to try and maintain that denial no matter how strong the evidence is to the contrary. It solves no one's problems. And that is to the detriment of both women and transwomen.