@Andy Burns
No, they are claiming they can develop thrust by removing energy from
the wind by slowing it down as it passes through their propeller, the
reaction pushes the vehicle forwards, which spins the wheels, which
spins the propeller ... I think!

Hi Andy,

Actually, the propeller never spins the wheels (it can't -- it's a
propeller and thus doesn't produce an output torque).

Initially the wind merely starts blows the vehicle downwind. Once the
vehicle is moving the wheels provide the input torque to turn the
propeller. The propeller takes wind that was moving at say 10mph
across the ground and pushes back on it until it is only moving say
5mph across the ground. The energy removed from the wind when it is
slowed is the energy powering the vehicle.