I wouldn't say it's a wide thing to say, it's about ensuring you are driving focus in to your selectors to prevent things breaking later.

Styling on element is just too open when styling components of a site.

Your styles should not need to be updated if you modify the markup of the component, using classes greatly reduces this hassle.

I don't think it's going as far as a guide on how to write CSS, it's just setting a good practice which will ultimately result in much more maintainable markup and styles.