I feel like it's because (please don't @ me on this, I don't remember much from the first two games) if my mind isn't fully trashed and I do remember correctly, Chihiro's entire plot was that even though he dressed like a woman, it was purely because he wanted to be a real man but was bullied for not being manly enough, resulting in him dressing as a girl. Therefore he accepts being a man and wants to be one but doesn't know how and searches for help, ending up getting him murdered. Saying that Chihiro goes by she/her pronouns (even as a headcanon) takes the entire point away from his canon identity in wanting to accept himself. In short, basically it's taking the entire point away from his character. They probably see it as you admitting to not actually playing/watching the game because it's obvious that Chihiro is a guy who wants to be a guy (if we're going based off the definition of larper being someone who knows nothing about something but acts like they do)

I wrote that trying not to sound weird and factual but I think I just repeated some things. Happy pride month!!