Because that truth requires everyone else to go along with it. People often compare it to being gay, but it's not comparable. Being gay is nothing to do with anyone else; if everyone at work turned gay tomorrow, you would never know unless they told you and it wouldn't require anything from you. It's nothing to do with anyone except themselves and their partners. It requires no reflection or validation from anyone else. It's an innate truth.
IW's truth, though, requires reflection and validation from everyone who meets or even mentions IW. It's not an innate truth like being gay (because it's not a truth at all). Some people may genuinely believe IW to be a woman (well they don't, but they'll never admit it), some might be on board with the truth but happy to deny it to be polite, but some people don't want to pretend and they aren't obliged to - and that's what's happened here.
JKR doesn't misgender people as a matter of course, but IW forced her to the discourtesy, as I believe the phrase goes. And at the end of the day, the right to tell the objective truth and not be compelled to lie must win out over IW's need for constant validation and submission from literally everyone else, which is where IW's "truth" lives.