I'm sorry, but I'd have to see evidence before I believed that.
My opinion is that a light flywheel doesn't affect the amount of torque an engine produces. OK, so the way that torque is delivered is changed due to the car dropping off RPM's faster versus your own ability to change gear fast enough
The flywheel is driven by the engine, so if anything I'd have thought power to the wheels would increase, just in the same way as improving other transmission components would have that effect.