More mass implies more angular momentum. The mass might have been built into the shaft to allow the car to overcome deformations on the path without overloading the engine with shock loads.
This is the lightest car outside of a Nash Metropolitan. The torque tube was used on Fords up until 1950’s and in Corvettes today, but in the Corvettes case it is because the engine is in the front and the trans is in the rear. The torque tube is needed to prevent any vibration in the input shaft. a micrometer at the motor is a millimeter 8 feet later. In the Opel’s case I have a 1 foot driveshaft (cue 1 foot driveshaft jokes)2 U joints AND a torque tube.