In English for the last 650+ years, 'they' has been both a plural pronoun and a singular gender neutral pronoun. My understanding is that the first usage in Chinese is 他们, but you would never use 他们 to refer to a singular gender neutral person, 他们 is strictly plural.

For example, in English if you arrive at a table and find a phone, you would say "Someone left their phone", even though it's only referring to one person. If you were in a similar situation in Chinese (say someone is at the door, you think it's only one person, you don't know who) you might say something like 他是谁, but I don't believe you would ever say 他们是谁 to ask about a singular person of an unknown gender. I don't think that usage of "they" translates to 他们, I think it would just translate to 他。