In attempt to clear some things up...
To me, an important thing to acknowledge is that trans women competing in sports and Hiltz's want for others to use they/them pronouns and respect their gender identity are two separate issues. Not all who support Hiltz's wish to be respected in the way they present support/are even sure about the complicated subject of trans women competing in track and field. I, myself a woman, do believe that trans women are women, but I am also incredibly unsure about them competing in Track and Field and think that there is an unfair advantage.
First, on Hiltz being non-binary. Though I will not speak for Nikki and how they feel about their gender, they have self-identified as non-binary and not genderfluid. Genderfluid means that they sometimes feel like a man and sometimes feel like a woman. To my knowledge, Nikki has not publicly said anything about this.
Second, most of this is related to language. All of you seem to be upset about the fact that Hiltz identifies as non-binary, not a woman, but is competing in a women's category. Come on guys... It just means biological female. They have not taken any testosterone or anything of the like, so they do belong in the women's category of competition.
Third, gender is a societal construct, as is language. Nothing in the creation of the world ever said that because someone is born with female genitalia has to be called a she or be classified as a woman, or any of this stuff. For hundreds and hundreds of years, Indigenous groups have had the concept of non-binary people, people who are just people, rather than being forced to conform to the roles of man or women. Many languages today don't even have gender-specific pronouns. It is not un-godly or mentally-ill to just not want to conform to a set of preferential rules society has put in front of you, especially not if others who don't identify with such constructs have come before you for hundred of years.
Another note on language... the singular they/them pronoun has been in use since 1345 according to the Oxford English dictionary. I will repeat that... they/them as a singular set of pronouns has been in use for 700 years, according to the OXFORD EN˝GLISH DICTIONARY, THE MOST DEFINITIVE DICTIONARY IN THE ENGLISH LANGUAGE. I am almost 100% sure you use they as a singular pronoun on a daily basis without even realizing it.
And if for whatever reason, that doesn't give you some understanding about non-binary pronouns in the English language, think about it like a nickname. Why is it so absurd that someone born as a she wants to be called they, if guys named Richard get to be called Dick, Roberts are called Bob, etc.? It's not that hard. It's respect. Please, guys, treat people with kindness.