Well, it is true they didn’t mention how a parachutal drag Cd (aka Ct in HAWT) of 2.0 was measured, I tend to believe it is correct, because predicted Ct for axial induction factors greater than 0.5 will not fit the theory of nice laminar flowing wind turbines - as in figure 5 in the overly mentioned paper.pdf When the so-called axial induction factor exceeds values of 0.5 empirical data confirms that Ct gradually increases towards 2 instead of dropping to 0.
Here-s how actual thrust goes vs. charted from Betz numerical derivation:
Image taken from the longer explanation here
The reason Betz computed thrust coefficient failes above 0.5 value of axial induction factor is that leads to negative wind speed downwind, which is impossible