raystuttgart
Civ4Col Modder
It is theoretically possible that a Nation produces 1000 yields per turn early on, because I think that is the number.Sitting Bull wins by industrialization victory in 1524.
But it is extremely unlikely that a Native Nation ahieves to do so. (It would need e.g. 50 cities producing 20 yields per turn.)
Summary:
Most likely a bug.
Without a save it is not reproducable.