This response will apply to Chaul Pelimo's post as well.
Hiltz says nothing about their biological sex (nor is it any of our business), which is distinct from gender. Gender is a social construct and a way of categorizing and making sense of the many people we coexist with. In the United States in 2023, the genders of man and woman (which are, again, distinct from the biological male and female sexes) entail expectations that those who are that gender will fulfill certain broad social roles. There is massive variation within how we think of "men" and "women," and the meaning of these terms change all the time, but these terms carry social baggage and indicate something about someone's identity. Most of the time, your biological sex significantly determines which of these social categories you will most identify with, and most of us choose to use the one that matches our biological sex. That is to say that most biological males identify as men and most biological females identify as women. There's nothing wrong with this! But not everyone's identity fits with the gender that most "naturally" matches their biological sex.
Hiltz does not feel they conform with either of the social roles of man or woman, so they use they/them pronouns to denote this. They say nothing about their biological sex, and compete with other biological females (most of whom identify with the social category of woman) because of this.
Chaul Pelimo, Hiltz does not claim "to not know she is a woman." You have no idea how it feels to be Hiltz, and how she feels with regards to the social category of woman, so you do not know anything about her status as a woman. You might be able to reasonably assume that she is biologically female, but she does not deny that as far as I know. You are ignorant. You can deal with being called ignorant. I am sure you will be just fine.
high school xc coach, for the purposes of athletic competition, Hiltz is grouped with and competes with biological females, most of whom also identify as women. She is not trying to decide her sex, she is deciding her gender identity, which no one can do for them. The fact that she competes with other women tells us nothing about who they are and what it is like to be them. Their identity matches with neither the concept of man nor woman, therefore they are non-binary, and use they/them pronouns. Their athletic status has little to nothing to do with that fact.
If you want, I can explain the concepts of gender and social constructs further. It's pretty simple, and I also like to let the simple things in life be simple. I'm not sure either of you would be able to understand it, though.