Do you have any source for this? From what I read of her, it seems she's criticized Labour for what she perceives as antisemitism. (Edit: She's also very transphobic though, apparently; that sucks, but it still has nothing to do with Holocaust denial. Edit 2: okay yes it does. What the fuck.)
My experience of Katy is an incredibly intelligent and balanced person. I can't speak to their history of antisemitism or trans activism one way or another though.
So I'm not understanding specifically what she said that's problematic in relation to the Holocaust? Did she say that trans people were killed during the Holocaust? Because that was a real, actual, systematic component of the Holocaust; saying that would be the opposite of denial.
Also, hardcore in what way? What's wrong with being pro-human rights in a "hardcore" way, however you're defining that? And where did she say everyone wants trans people dead? And again what does that have to do with the Holocaust?
Wait, who's Katy in this scenario? I read this as "Rosie" initially, because I couldn't think of anyone named Katy related to this and so my brain autocorrected.