Stability, army maint, fort maint, merchant placement, war reparations, vassals - basically any number of things could make the difference. May also simply be a rebalancing between DLCs/release versions that saw the Incas advantaged when the videos were made but not any more.
Actually, given that Inca aren't a starting nation, they've likely done a bunch of production development on gold provinces. That's probably the difference but to upgrade development you may need the Common Sense DLC (can't remember if that's a free function now or not)