Page 25 (in French language and two diagrams), the S structure of the Savonius rotor is shown:
Among other things, the S-shaped structure is characterized by the space between the two half-cylinders which allows air to circulate and also exert force on the other half-cylinder which returns upwind. This is shown on the diagrams.
A Savonius rotor without this space is not really a Savonius rotor. And perhaps a so high Cp of 0.3 can be explained with this feature when the realization and the proportions are suitable.
The same diagrams on