As long as I’ve been alive, the pronounds “They/Them” have been reserved for unique instances. It’s been used as a plural, a general non-specific pronoun, singular indefinite antecedent, and to protect an individual’s identity. These instances have been pretty codified in our minds from school. It was always incorrect to use “They” to refer to an individual that can be addressed with “He/She”. Historically, the structures that we establish to define our world have taken a lot of time and consideration before catching up to what’s been accepted as the norm. As time goes on, that gap narrows, and our knowledge body adapts faster than ever. If you check baseline dictionaries these days, such as “Oxford English” and “Merriam Webster”, you’ll see that along with the usual plural, indefinite antecedent, and anonymous referral, they’ve added “They/Them” as correctly identifying people who are non-binary. The gap has officially closed, grammar has caught up with popular reference, and you can’t back out of this extra set of pronouns by using grammar as an excuse.
I personally had no idea, so I looked it up today. And despite being nearly forty years old, having been ingrained my grammar skills by an education system that’s nearly thirty years antequated by this point, I can still adapt. I can concede that “They/Them” isn’t invalid due to language limitations. That’s what our language does- it adapts to our use cases. It always has. The only difference is that now it’s doing so at a far more rapid rate than ever before.