He didn't mean the planet, but that answers his question. The people that used to live on Zeffo was more obscure knowledge, from his understanding.

"Yes. Named after the people that lived there a long time ago. They were Force-sensitive and disappeared mysteriously a long time ago. Master Cordova was very interested in them. It was one of their ancient sites, a vault, that gave him the vision of the fall of the Order. When he realised the Council wouldn't heed his warning, he created a holocron with a list of Force-sensitive children and hid it inside the vault. After the Purge, Cere made it her mission to retrieve the holocron."