I think that Nikki finds themselves in a catch 22. Because there's no logical argument support trans women competing with cis women in sports, while also supporting themselves competing with cis women.
Nikki does not identify as a woman, nor do they identify as female. They discussed taking HRT and testosterone as well as having top surgery after their career is over.
But Nikki does not take any issue whatsoever with competing alongside cis women. They have never advocated for a separate category to be more inclusive of non binary people. They have never insisted on racing men. And the question remains: why is that? If gender identity is this monumental, why then, is Nikki fine with standing side by side racing and training with women?
The answer, (I believe) is that despite being nonbinary, Nikki was assigned female at birth--and I say that not believing that an assignment of female is arbitrary. Nikki has a "female" body and so it makes sense to them to race others with the same body. But if they were to state this, then they would be in direct opposition to trans women competing alongside cis women.
I don't have any issues with Nikki competing against women. What I have issue with is their continued advocating for cis women to give up their spaces and rights and the fairness of sport.