"Pick on" is a pretty light-hearted term for what was done to trans people during the Holocaust.
As for if her comments are Holocaust denial or not, I think an argument can be made that it is. The destruction of the materials was the tip of the spear that imprisoned and murdered many "sexual deviants" in the camps that would come after the burning of this material, including trans people. The "gotcha" that is being put forward by JK in this case is that Hirschfeld was a proponent of eugenics (which is true), and that those Hirschfeld worked with in turn helped the Nazi regime with immoral experiments and sterilization (also true), including in the camps. But she explicitly points to threads that claim that
because of this connection, trans people were not a Nazi target when clearly they were. The whole thing hinges on pushing the idea that
Hirschfeld was somehow influential in the Nazi eugenics program, and that influence would mean that trans people were somehow ignored, and that it was really just the homosexuals that were tortured at the hands of the regime. It's erasure of what was done to trans people during the Holocaust. Is that not denial of what the Holocaust did?
The mind boggling thing to me is that Hirschfeld was seemingly okay with forcible sterilization of "degenerates", but didn't see the connection that many saw people
like him as a degenerate until he was in the last years of his life. That said, Hirschfeld did wind up writing a book exploring racism and noting that he was "numbered among the many thousands who have fallen victim to the practical realization of this theory." He went further to suggest that the Nazi style of racism was simply an extreme version of the prejudices held throughout the west, calling for the end of the use of the word 'race' to describe subdivisions of the human species. This sounds more like someone who by the end, realized what was up, and spoke out about the wider racism/bigotry at play. And he spoke up as the Nazis were coming to power and instituting their earlier policies, as he died in 1935.
So it seems strange to pin this on him or suggest a link that likely doesn't exist, as Hirschfeld would have likely been appalled at what Gohrbandt had done going by his later works.
https://Twitter or X not allowed/jk_rowling/status/1767928717538644460
https://Twitter or X not allowed/jk_rowling/status/1767939048427896900