How long has the concept of "they" as a gender neutral singular pronoun been in existence; aka how long has it been in use in the manner described above?
I don't know the exact date at which its use became common in the english language, but as a native speaker, I can tell you that this is the natural way to refer to anyone whose gender you don't know or a hypothetical person in an abstract statement. At least, this is the case in Canada, where I am from, and I am pretty sure it is the same in the US.
As an example, if I met someone and they told me they had a child but didn't tell me whether or not they were a boy or a girl, I might ask, "how old are they?" I wouldn't have to think about this at all, it would just be the natural way to ask the question.
I will have to disagree with the use of "they" to clarify one person. It makes no since as this word is used to describe what multiple are doing / where they are. It seems to me like this is a pointless conversation to even go about as you would describe it just like this " There is 1 person working on the mod". I would rather have their gender wrong them seem like i don't know the English language by calling one person "they".
Honestly it doesn't matter what you call him by anyway, i'm pretty sure the Author doesn't care what people on a gaming forum mispronounce his "gender term".
Actually, you sound like you don't know english very well when you are adamantly opposed to the use of the singular they.
I understand your frustration with overzealous social justice warriors, but you are picking a fight with the wrong pronoun. The use of the singular they is not new. Every native speaker I know uses the singular they.
Edit: Just thought I would add that using plural pronouns for singular words is not limited to "they". "You" is actually a second person plural pronoun in origin, not a second person singular pronoun. The original second person singular pronoun was thou, and only in the last couple hundred years has it been completely replaced. If you are opposed to "they" being used for a single person because it makes the number of people confusing, then you should also be opposed to "you", because it can refer to a group of people or a single person.