DevTools: Box model can be modified even without the modified properties set in the css file?

In this article / exercise it has this note:

Note: If you inspect an element and find that the border is set to - , adding a numerical value will not make a border appear. The border color, style, and width must be set in the CSS document in order to see the border.

However, I tried modifying the dashes and the changes were in fact reflected on the webpage as so: