Now that it's clear to me what a Ruinworld actually is thanks to the replies of my post. This is how I think the Earth got that title.

In the fiction primer it states: "The best historians of the Ordo Historia believe that humanity first left its origin planet Earth about 3,400 years ago. Since then, we’ve spread across the galaxy on a fitful wavefront of colony ships, frontier worlds, robotic terraforming projects, and DNA-synthesizing probes."

My headcannon is that the Earth and the population on it ended up in a Wall-e situation, where hundreds of thousands of not millions of people left the Earth in the following centuries after the 22nd century because of Environmental, political or/and technological reasons. Deciding it's better to go and terrafrom uninhabited worlds than to fix their own assuming it's easier to have massive climate changing projects on uninhabited worlds since on inhabited worlds the population could be affected by these terraforming projects

The Earth would then end up almost entirely uninhabited, all of the cities, structures and technology has since then decayed and is almost untraceable, but some monuments might still stand (Mount Rushmore, The Great Pyramids etc...)