
I know this can be read as "I can call myself whatever I want for any reason!", but I don't think that's what she's saying and there's an actual point behind it.
Defining "biologically female" by genitals is arbitrary (that's what "genital essentialism is a social construct" means). A person who was born with a penis but transitions, and e.g. has oestrogen levels typical of an adult woman is now "biologically female" in at least one relevant sense, while still being a trans woman. I'm pretty sure that's what she meant, anyway, it's a standard argument in trans circles.