That's true, and there is evidence of "they" being used as a singular as far back as over 700 years ago, but only within the last few decades has it been formally accepted by style guides, like the APA or the Chicago Manual.
And that's actually a pretty recent development. Less than a decade ago, I remember getting marked down in English class for using "they" as a genderless singular pronoun, as my elderly teacher grew up only ever using "they" to refer to a group.