Okay, I agree that J K Rowling is not the British Prime Minister, or a member of the British Cabinet or even a Member of the House of Commons or a Peer, but she does have a platform.
When it comes to transwomen being able to be called women, I equate it with an argument I had in my first year of university with a self confessed racist, who, when we were talking about what it meant to be Australian, and they talked about Fred Hollows, I mentioned Victor Chang. Victor Chang was born in China, raised in Hong Kong, and after his mother died of cancer, completed his secondary schooling in Australia and went to university in New South Wales. He also practiced in Australia. The self-confessed racist told me that Victor Chang wasn't Australian. Well, sorry, but he had Australian citizenship, and his parents were Australian-born Chinese, even if he was born in China.
Yes, a transwoman retains the XY chromosomes and what is needed there is a sensitive doctor who says, "Okay, as you age, you will need to have your PSA checked." But a transwoman is not a man.
My point here is this, the self-confessed racist said that Victor Chang (I mean the famous heart surgeon and the late Princess Diana was a patron of his Foundation) wasn't Australian because he wasn't white; a transwoman is a woman even if they have a penis and a transman is a man even if he has a vulva.
What people need to remember is, a pre-op transwoman, who has been on hormone therapy, eventually cannot obtain an erection, so if they cannot obtain an erection, they cannot perform. The penis pees.
I think of J K Rowling a bit like this. I went to Japan, when I was 21, and on the flight back to Australia, I encountered two couples who had been to Canada but had flown Japan Airlines. They might have flown Japan Airlines not because they wanted to see Japan as well (they had a stopover in Tokyo for a few hours), but they might have flown Japan Airlines because it had a better price than QANTAS. Similarly, though, you can like Harry Potter but not J K Rowling, the same as you can drive a Landcruiser Prado because it's a good vehicle to have, rather than because it's Japanese.