the tl;dr is that they are not really feminists, and they use feminist-sounding rhetoric to justify anti-trans views, they're basically just anti-trans activists.
For more rabbit-hole, ContraPoints has a video about "Gender Criticals" (synonymous with TERF): https://www.youtube.com/watch?v=1pTPuoGjQsI
Her video on JK Rowling would be a good follow-up.
Since I transitioned I've been thinking a lot about how little I knew about trans people until I realized I was one and then took much more seriously educating myself.
It makes me feel ashamed because of how little I understand so many other oppressed groups, and how little true empathy I have. Even if on the surface I have respect for people and consider myself an "ally" to various groups, I feel I should do more than just signal respect and support. Maybe it's an unrealistically high bar, but my conscience certainly thinks I need to do more to empathize with and better understand other groups.
I can't help but feel my default tendency is towards a kind of accidental tribalism - I understand perspectives I choose to engage with and understand and this results in a cultural cloistering, an accidental in-and-out-grouping because of how I naturally do or don't understand someone's life experience based on my own. Unless I go out of my way to do a lot of work to understand other perspectives, I otherwise won't be likely to do that.