Well, thank you for explaining. I have complete respect for anyone's choices in this area but I'll leave the opinion on this particular choice to myself.
Edit: looking at other comments, it seems like it might not be the correct explanation after all?
As someone else noted, GP is just misunderstanding a frankly confusing subject. There are essentially two common ways for people to specify their pronouns.
Some people desire a single set of pronouns, and indicate this using the nominative/accusative pair (she/her, they/them, ze/zer).
Other people accept either of two different sets of pronouns, and indicate this preference using the two nominative forms separated by a slash (she/they, they/he, and apparently, it/she).
Edit: looking at other comments, it seems like it might not be the correct explanation after all?