> Looking at the google example, they are setting the font color. It just doesn't look good when dark-mode CSS doesn't take the existing colors into account.

Did you read the accompanying text? What you've said is literally the exact opposite of what is happening.

Google's CSS is not taking the default system colours into account. All of these sites seem to be assuming that the default system colours, implemented by the OS, are black text and white backgrounds. There is no reason to assume this, and indeed this is not the case on a minority of machines. The author's system happens to use white text and black backgrounds. So do I. Others are using low-contrast grey-on-grey themes, still others have a variety of multicoloured themes.

The author is trying to spread awareness that this seemingly common assumption about default colours is wrong. Hopefully this awareness has reached at least one person (yourself) ;)