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 in the english language 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