However it is a quote from @floba’s publication, page 2.
The more complete quote is:
Because the power increases cubically with CL and decreases only quadratically with CD,eq, moreover whose major contributor usually is the tether drag, the following Hypothesis was made in this study:
Hypothesis. Maximizing the main wing’s airfoil lift coefficient to or close to its physically feasible maximum, also maximizes the power and energy yield as well as the allowed costs and profit margin of a drag power kite.
I think this is true at least because in the simplified Loyd’s formula : [P = 2/27 aD A w³ CL (CL/CD)² ] , the coefficient of lift (CL) works one factor more.
In practice tether drag is a limiting factor, so that a very high L/D ratio for the kite alone is not an essential parameter. So by taking account of tether drag a very high lift coefficient could lead to a higher efficiency (beside it, a theoretical example of Magnus cylinder in crosswind maneuvers provides a very high value of the lift coefficient, and a relatively low value of the squared glide number).
I think the higher aspect ratio of the biplane configuration leads to a still higher lift coefficient. But I am not sure this feature is fully optimized in this design. I think a balancing between the aspect ratio and the area / span ratio would be to find.