So Betz assumptions are valid as long as the wind speed at turbine level is no less than half of undisturbed wind speed, and since the maximum Cp is reached at 2/3 (a = 1/3), nobody cared about the a > 0.5 zone because negative wind values are possible only in turbulent regime and that means even more energy lost than predicted mathematically for laminar regime a > 0.5
I think that where the Brazilian paper fail is by labeling regime above 0.5 as “impossible” and ignore it instead of simpler non-solution - Betz assumptions do not apply there therefore we cannot make meaningful conclusions about that regime applying the same maths Betz used for static wind turbines.