It's not absolutely established. If you read beyond the introductory paragraph, you see that it is indeed in debate among many groups. Yes, it's been used colloquially for a long time. I'm aware of that. But that doesn't mean it does properly within the syntax.
In fact, most style guides (as referenced in the very article you linked) recommend restructuring a sentence to avoid the singular they issue altogether.
That being said, I'm not opposed to using it in everyday speech if you're legitimately unsure of someone's gender. It can be a simple shorthand and is certainly more respectful than the abhorrent "it." But what I was trying to get at was that I don't want it to be used because someone refuses to call someone by their identified gender.
I'm trying to be nice and encouraging, so I'm not sure why I'm getting pushed back against for a small side comment.
I'm dragging this up again because I was busy with a French final, but ya'll may have noticed that I said it was subjective because I was acknowledging (and not shying away from) the debate over it in recent centuries.
And you're getting pushback because you're being pedantic against uses of the language that are long established (style guides can, do, and will argue against such things) and that actually empower some people. I'm not going to insist on being a pedant and dictate to NB people or women that they must use a generic
he (this is the corollary to not calling someone by they/them who doesn't wish for that). Use defines language, and you can argue about what
ought to be employed, but slavish adherence to syntax and style declarations is to throw your mind into the sea. Adverbs are railed against in style guides too, but that's not to say they're absolutely proscribed.
You can clutch your pearls about what's proper. Me, I'll stick to what's effective and what works.