I think it is because the radial traction towards the VAWT-Flettner balloon (or any wheel) prevents rotation. In contrast, the traction towards Kiwee is an axial traction, after transmission to the right angle idler pulley. Similarly the traction towards Keuka wind turbine (or any regular wind turbine) is an axial traction, allowing torque transfer through rope-drive cable surrounding this rim-driven wind turbine, as shown on the video below (from 1:37):
The rope-drive cable could perhaps be replaced with @Freeflying 's annular generator.