Feminism is about upholding the rights and dignity of all women – not policing the boundaries of womanhood. There are no ‘sex-based’ rights, only human rights.
It literally just means ‘not trans’, and there’s nothing wrong with not being trans – just like there’s nothing wrong with being trans. The important thing is to be yourself,* because there’s nobody else who can be you.
*Unless you’re a bigot. If you are, you should try to change that.
If you have made a lot of money writing wildly popular children’s books, you can use your wealth to benefit society and the natural world. You don’t have to spend it on promoting hatred!