You got it. A swaybar DOES add to the spring rate when the car is cornering. That's exactly how an engineer looks at it, they calculate the equivalent spring rate of the bar in torsion. But, as Bob said, the bar doesn't support the weight of the car. When the bar in its static condition (or when BOTH wheels hit the same bump, such as a speed bump or RR track), it does not add to the spring rate because there is no torsion or twist in the bar.