He specifically mentioned the Nazis burning down the Institute for sexual research in Berlin a research facility which treated transgender men.
I've noticed the TRAs have really picked up on this recently (not saying your DS is a TRA btw). It's as if one of them read about it, thought "Aha! Evidence of transgender persecution by the Nazis!" and it spread all over social media. In fact, one of our regular visitors who used to "flutter by" was a big fan of referring to this in a way that suggested trans people were the main focus of the studies at the institute.
In fact, the institute studied many aspects of sexuality, of which transsexuality was just one. The institute was denounced as Jewish, Social Democratic and offensive to public morals. Hirschfeld was Jewish and gay, so an obvious target.
He was also an advocate of equality for women. That bit always gets left out!
JKR is not a Holocaust denier, although perhaps unwise to comment on the issue because she must have known how it would be taken out of context. But she can make her own decisions. As far as I understand it, she was saying the Nazis did not make trans people a specific main target for genocide. Which is true.
To me it sounds like TRAs attempting to rewrite history again (like the Stonewall riots) and place trans people at the centre of the Holocaust.