The essay doesn't really say anything about when the singular "they" was invented.
What it says is that it used to be low-status and unsophisticated language.
> In the 1970s, fancy people would have sniffed at using “they” rather than “he” for a single person of unknown sex like this. But today, fancy people would sniff at not doing that. How did that happen?
> I think “they” climbed the prestige ladder—people slowly adopted it in gradually more formal and higher-status situations until it was everywhere.
The essay’s narrative is overly simplified and misleading about details. Singular they has been common for centuries. The idea that it was lower status is a more recent invention. The author might be referring more to use of they as a personal pronoun. Anyway, whatevs, language changes, that part of author’s message is good.
Yeah, I'm old enough that I remember being taught not to use singular they in elementary school, but I'm young enough that he/she was given as the preferred alternative.
> In the 1970s, fancy people would have sniffed at using “they” rather than “he” for a single person of unknown sex like this. But today, fancy people would sniff at not doing that. How did that happen?
> I think “they” climbed the prestige ladder—people slowly adopted it in gradually more formal and higher-status situations until it was everywhere.