It's so odd, isnt it? If they accuse someone of something which is apparently so obvious, you would expect damning quotes to be there at their fingertips.
If you wrote 'find me something racist Marine Le Pen has said' or 'find me something sexist Piers Morgan has said', then even if the person could not recall their words instantly, it would take them less than five minutes on Google to find something.
All they can do with JKR is waffle and handwave and say she is clearly transphobic because, look, she is, etc. It's utter nonsense.