“You only moved the headstones! “
Paris has a history of moving cemeteries. As the city grew they moved old cemeteries further and further away to make room for housing, and for fear of health (plagues etc). If you ever visited the catacombs of Paris - it’s a bit macabre - it was a limestone mine but eventually around 1780 the city decided to move the buried inside the mine to free up the cemetery land for development. There are over 5 million skeletons down there, and you can walk and tour rooms full of heads, spines, femurs etc. All neatly stacked against the walls. Millions of them.
So it would’t surprise me if they found houses built on top of old graveyards going back even further. The city is about 2300 years old.