Except Siz hasn't done anything except not really speak against him. She has acknowledged his evil. He hasn't really even tried to manipulate her since May (that I recall. I may be wrong). He certainly didn't in this last goodbye message.

I see where Hydra is coming from, but she is lashing out somewhat indiscriminately.

ETA: I also think that there may be some resentment towards Siz for not putting herself in the actual line of fire. Yes, what she was doing and has done was vitally important, but except for Rookwood and that battle, she was mostly safe. I remember someone commenting on how during the battle of Hogwarts she 'hid' in her tower. Of course, she was protecting many, many students there, but it was a comment.