Amber probably isn't any more sincerely transphobic than she has sincere trypophobia. She's catty as hell so she uses the wrong pronoun because she doesn't like that Kasey fired back in a video, sending Amber in a tailspin that made her delete her own bullshit rape story, ignore all questions, and if I'm remembering correctly she went silent for a few days because she knew she fucked up big fucking time. That's her way of being covertly passive aggressive in the face of defiance, to use the wrong pronoun, because that's all she can secretly do right now with everyone watching without staring another fire under her own ass.
Kasey has shown himself to be a respectable person who addressed and handled the entire situation in a very mature manner instead of screeching and crying into a camera, then going ghost and acting like none of it happened. Amberlynn I don't know is genuinely transphobic, maybe she is, I think she just hates Kasey. There's screenshots of her asking him to take the video down somewhere so she knows she screwed up royally, and that he hasn't backed down and has spoken out probably absolutely makes her crazy angry. Any normal person caught in a lie like that would cut their losses and try to bury it but she's just extremely vindictive and petty and doesn't know how to quit while no one's looking.