I don’t read that as “JKR demands a source for persecution of transgender by the nazis” and more as a “JKR demands a source for her upholding gender ideology of the nazis”.
At the very least Alejandra could have asked for clarification before throwing “holocaust denier” in the ring.
But I also admit that I don’t actually follow this very closely, so it’s possible I’m wrong and JKR meant to state there was no persecution of transgender.
Again, a follow up question would have clarified that. But both sides, her, her supporters and the people who attack her are so deep down that cycle of outrage that they are not really interested in understanding where the other person is coming from. They just want to be right.