I understand there are sometimes when you use they/them to refer to a single person, but it's when you aren't sure what the gender of the person is. And I don't mean in the sense of "Oh but even they don't know their own gender!" I mean like if you were to describe someone who stole your wallet or something but you have no clue if it was a man or a woman then you would use they/them because no one knows the gender of the thief.

I call this usage the "one-of-many" they, which is when you use the pronoun to refer to a single person who you haven't identified yet, that could be one of many possible people. Even when you know the gender of the hypothetical person you can still use "they" in this scenario.

The usage that nonbinary people are trying to push is "one-of-one" they (where it is someone you've identified), based on not making a distinction between the two types of singular they. Finally, "many-of-many" they is the plural one that nobody disagrees about.