[sgt]

Because it's grasping at straws. A singular case of a poem referring to an unnamed person? They are trying to make it seem like this was likely common and accepted 700 years ago up until today. I really doubt the author considered gender identity. In fact, up until quite recently, the use of "they" as gender neutral would cause blank stares.

[klodolph]

Why would it be necessary for the author to consider gender identity? I don’t follow the logic of your argument.

The cited example is just the oldest known example of singular “they”. You’ll find more examples. You’d have to rip a lot of pages out of your history books in order to come up with a timeline where singular “they” wasn’t natural English. All that is happening now is that “they” is being used more often to refer to specific, known people, which is not much of a stretch.