These people think she's not fuckable so therefore she's not a woman, and furthermore think women's sports are worthless unless they see the athletes as fuckable. That's it. It's patriarchy with a new coat of paint.
There is no trans woman in this case. A female boxer, born and raised as a woman, is being attacked and called a man. What OP is saying is that it's basically because they don't see her as attractive, and that's definitely a major component of it.
What it comes down to is that they're so obsessed with the idea of trans women that they start seeing them everywhere, and it usually comes down to someone not being quite feminine enough in some way, though there have been cases of people trying to claim some of the most conventionally attractive women in the world were secretly trans.
Ive always had this feeling, increasingly validated lately, that there's some weird push to invalidate all women that the right doesn't find fuckable.
There's all this coming after women for having T, and before the Olympics there's been a rise in this "if you think this woman is attractive (generic female athlete), you're GAY" Content.
It's really obvious when they're having a go at cisgender white women about makeup or certain clothes or haircuts or body types. You always see the same comments explaining to women that they ackshually prefer 'natural looks' or less revealing clothing or long hair or 'something to hold on to'.
It's all through a lens of 'why do you make choices about yourself that don't improve your appeal to me, someone who wants to fuck you'.
That there are women that completely reject all those notions or don't seek their opinions (women who are trans, women who aren't hetero, women who reject traditional feminine looks, women who excel in their careers or physical sports) simply doesn't compute with these people's view of what women are.
The more malicious parts of that mindset lead to that push you're talking about. We as women are still just something that needs to validate its existence. Any deviation from that is viewed as something that should never have existed at all.