In Galactic Civilizations IV why is Ruined Ringworld orbiting a planet, not a star?

In Galactic Civilizations IV mission line about the megastructures, civilization discovers ruins of a ringworld that are still partially functioning and habitable. However, ringworlds generally orbit stars (as do all functioning ringworlds in the game) and this ringworld orbits a planet. It orbits a star, looks like a planet and is identified as a planet. How would you explain it? It is especially important since I plan on paying homage to this in my story and I need to understand this. 

Image source: Galactic Civilizations IV, private playthrough.