They/them is used when referring to a single person when their sex is unknown or is meant to be deliberately vague because it's not relevant to the story /could detract from it.
An egregious example would be:
>We found their body over here, sir
>damn it jenkins that's the fifth one this week, where's his body?
>This time it's a woman, sir.
>Oh. LMAO. Who cares. We have to catch this damn serial killer, he's killed 4 men this week.
Neither of those cases apply to pronoun usage. It's not unknown, you (and everyone else) knows what the the person is. And it's not like you're being coy by introducing yourself as a vague concept/person. I'll use they/them when I'm referring to friends as a one off in a story, when I don't want the focus to be on them.
>So I was hanging with someone yesterday, had lunch with them, gonna be late to dinner.
>Cool
(would be said to a jealous gf, for instance, who you don't want to be worried you're having an affair just because you're meeting some girl for a business lunch)
>Oh? Who are they?
<she didn't gloss over it
>she's a sales manager from a company we're trying to seal a deal with.
I.e. you and everyone else who knows / cares knows it's a girl.
Using they/them deliberately is a way to sidestep some nebulous disadvantage associated with your sex, like women and competence, so they say they're they (lol good luck ESLs) because "I'm not like those other women, I'm a they!", ironically proving the point because no amount of avoidance will allow you to actually escape from your sex.