I guess I can understand why you are asking. That said, I think it’s a misunderstanding based on something that often comes up—people see explanations of social concepts working like mathematical equations. They don’t realize that they’re more simplifications of complex concepts, and that every statement will be some level of generalization.
Instead of trying to synthesize information from different statements, I think it’s more useful to look at the broader picture. Hate speech is a term that is used to mean something worse than simply disrespectful. It’s more about using speech to belittle, advocate discrimination, or worse to someone in a suspect class. The use of “girl” could be used that way, but it wouldn’t be the default.
More often, it would be a microaggression, a small thing that wouldn’t matter much if it was only one person saying it, but when it’s said by a lot of people, it starts to get hurtful. And that, if a lot of people say it, it shows a societal issue with the topic at hand.
I do try to avoid using it, but I’ve slipped up when thinking about college-age people, because, as I get older, they seem more like kids to me. I remember, when I was 18, I wouldn’t even refer to teenagers as girls, but instead “young women.”
Also, it doesn’t help that the female countepart to “guy” is “girl.” No one uses “gal,” which I think may even have just been a different pronunciation of “girl” anyways. There isn’t a casual term that implies female-ness.
My point in all of that it is that it’s too easy for people who are wellmeaning to slip up and say “girl” for it to be considered full on hate speech.
Of course, since opinions vary widely, I wouldn’t be surprised if there are at least a few who would call any use of “girl” for adult women to be hate speech. But I would consider them on the fringes.