They can be singular or plural. They is not "gramatically incorrect" if that is what the person is using as their pronoun, for example someone who does not call themself "he" or "she."
Zaboem is flat wrong, and that is the single most irritating argument. I'd call it a "pet peeve" but honestly its more than that. It most certainly strikes a nerve, as I have several friends who use they/them as their pronouns.
Language is always changing. English is no exception. When language fails to function, we invent new words or expand their meaning. No language is stagnant except the dead ones. Further, when you don't know who did something, you say they singular. It already exists as "grammatically correct" and saying that the singular they can't or shouldn't be used is bullshit.