It’s in TFA, just a few sentences in.

Isn’t the article about how to best deal with the UI for letting the user indicate that the system should decide? Which covers your case.

Thanks, I missed that! But then I don't understand why a seasoned designer purposely suggests that ignoring it might be good for the user, especially since light/dark theme toggle is for a small fraction of the people who genuinely care about the theme being "right for them" and not just whatever.