The existence of gender identity is absolutely provable. Your gender identity and your birth sex are verifiably not the same because girls aren't born wearing dresses. You have a uterus because of your birth sex, but you wear dresses because of your gender identity. You wear dresses because of who you are, your identity within society, specifically the aspect of your identity relating to your gender. Your clothes, your makeup, your mannerisms and behavior, every aspect of you that exists only within the context of other people, that is your identity. If every other person on Earth disappeared tomorrow, your gender identity would effectively cease to exist, but your birth sex would be unchanged.

JK Rowling posting pseudo-intellectual, pseudo-inclusive gibberish as per usual. At least try to understand the words you're misusing.