I think they mean using CSS `prefers-color-scheme` the user can set a light/dark preference at the browser level and your site doesn't have to have its own implementation. https://developer.mozilla.org/en-US/docs/Web/CSS/@media/pref...
This would still remove the visitors ability to say "I want THIS SITE to be in light/dark mode", without affecting all other sites / apps on their system. This page could support reading from the browser/OS, while still giving the choice on top of that.
Again, this choice was probably implemented ... to give the user a choice.
Some people may not want everything to be light or dark mode. I am one of them.
Some articles may not work well with dark mode.
Like someone above said, some sites implement it as an after-thought. In my case, I'm giving more love to dark-more because there are simply more nice color combinations in dark more than with white.