It just seems incredibly odd for there to be so many lines in a book about gender insisting that there is no way to refer to someone (in the English language, at least) without implying gender. She even mentions the possibility of using „it“ at one point!
I’m liking the book otherwise, but every time the narrators ponder about pronouns without even considering „they“ I have to ask myself if there is any point in ignoring it or if she genuinely just forgot. I don’t think it’s possible for her to have not known about it considering how well-read she was and how long it’s been in use.
To the best of my knowledge it would be correct to say that English language style guides at the time treated they/them as strictly plural, but that was actually a more recent change (ie, within the last century or so). Singular they existed for centuries, only to fall out of style with the increasing formalization of the English language. It’s use today is not a modern invention, but a return to its original usage.
So yes, Le Guin was probably taught that it is not proper to use it as a singular, and that’s how anyone alive at that time would have been taught to use it. But that’s not the same thing as saying it was “always” singular.