I don't know, the sentence "by 2020 most style guides accepted the singular they as a personal pronoun" doesn't scream "it's been used like this for centuries" to me.

This is one of those things that's both true and false at the same time.

It's true it goes back to 1300, but it had also fallen out off fashion and was considered "wrong" later on. Languages change, and don't do so in a linear straight-forward way. From our perspective, it's very much a neologism (although it's been a few decades, and arguably already passed the neologism stage).