That's as easy as it sounds. Every province in EU4 is flagged by "discovered_by = <some tech groups> ". For provinces outside CK scope, we don't alter this. For provinces inside CK scope, since everyone saw everything anyway, we flag them all as being discovered by everyone. How exactly would we make a distinction, and where, when CK saves can produce utter chaos and cultures can be anywhere across the map? And tech groups are tied to those cultures, so visibility would be utter hodgepodge. It's far easier and makes more sense to unlock CK map to everyone in EU4.