Indeed.
And the blades of a Savonius kite would cover all the area (unlike Magenn), aggravating the digging of the blades going up the wind, and therefore the air drag.
I think it is also the reason why the power consumption is so important with Flettner balloons, probably still more with high winds.
Flexible materials are inferior in regard to aerodynamic features. It is the reason why the blades of current wind turbines are rigid. But AWE faces to the weight issue.