Actually I don't think the explainer gets into the full story. The reality is it's not CSS's problem. It's the browsers that have historically made text scaling weird on each platform that they support.

And now just like with the viewport meta tag, we need a meta tag to say, 'Stop doing that please. Make the default font size in my CSS work the way it always should have'.

The other reason why the flag can't be in CSS is because it needs to make em and rem units in media queries get affected by the user's text scale. See the explainer for more info on that.