you use singular they all the time
someone’s knocking on the door, i wonder what they want
there’s a person waving their hands at me from over there but i can’t hear them
does mysterycop follow you? i love their username
can you tell whichever person was in the bathroom last to please close the door behind them on their way out?
they IS properly, officially used to refer to a single individual person, quite a lot, in a plethora of different scenarios
all the opposition comes when it’s suggested that you can still use they once you’ve seen the person or heard their name, because you know someone’s gender once you’ve seen them or heard their name, of course
the opposition is not grammatical