Perhaps the IMA motor is used for auto stop / start, which might explain the smoothness, but a cold start (from key-off) probably uses the starter motor; the article needs to be updated, otherwise it's very informative!
The conventional starter is there in case of a failure of the IMA system's battery. Also as a back up for cold weather operation. In normal operation, the IMA motor starts the car. IMO, the 12v battery is also there to run all the car's electronics since they're already designed to run on 12v not the higher voltage that the IMA battery pack supplies.