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.
The core idea of radical feminism is that humanity is divided into two groups - male (ontologically evil, sexual predators from birth) and female (weak, incapable of harm, in need of protecting)
while some radical feminists define these groups as being based in assigned sex, and others define them by gender identity, it's the ideology as a whole that is transphobic, and harmful in many other ways
It is far from typical feminist beliefs. Thty advocate for things such as female seperatism, and killing all those they consider men or culling them to a small population to be kept only for breeding.