there are many possible reasons for that:

• she might have thought that the general critic on gender roles is allready enough for the reader
• she saw singular "they" as something for written or formal language and not everyday talk, which is what most of the dialogue in the book is.
• she was not happy with "they" since it can lead to confusion around singular and plural or did not see it as better alternative than a generic masculin pronoun for someother reason
• she simply did not use "they" that way when she wrote the book.

I sadly don't own the anniversary edition, so i don't know if le guin elaborates further on that. It's definitely valid criticism however.

e: english grammar hard.

do you think Newton was an idiot because he didn't know about relativity?

you do know, that the majority of modern gender discourse... originated in the 1970s, and only became popular with non-academics in the in the 2010s, mostly thanks to it's use on social media?

but don't let these facts get in the way of how much a book written before most of any of what you are talking about, existed, let alone outside of a tiny tiny subset of academics who were originating most of what you take for granted as having always have existed?

perhaps you could, I dunno, give Le guin credit for what a trailblazer she was in her time?

^

Didn't read the book