But here you only have one fan, the propeller, and it obviously the same size as itself and is pushing directly back at the same wind that is powering it. So first, I don't see how your 'gearbox' analogy works and second, you didn't respond to the question of how the wind and the 'artificial wind', which you claim can be stronger, can coexist in the same space.
On blackbird the vehicle is like a sail at initial start so wind pushes against the vehicle body and the propeller blades (much smaller area then when the propeller rotates). This power is split between accelerating the vehicle and powering the propeller by taking energy from the wheel.
So if you disconnect the propeller from the wheel then vehicle in same wind condition can accelerate much faster and get much faster to some speed below wind speed.
But instead of using all wind energy to accelerate fast you divert large part of the energy to power the propeller that creates a sort of artificial wind eventually at multiple times the wind speed.
So wind energy will be stored in vehicle kinetic energy then in propeller kinetic energy (spinning mass flywheel) and finally in pressure differential energy storage.
As you start to put more and more energy in the pressure differential energy storage that starts to also get used by the vehicle but even this is put back in all this 3 forms of energy storage the vehicle kinetic energy the propeller kinetic energy and also very important propeller pressure differential.
But when this pressure differential is the main or even only energy source it is put back in to kinetic energy of the vehicle and propeller kinetic energy so propeller spins faster but because vehicle also move faster the pressure differential drops.
When pressure differential gets so low that it can no longer power the vehicle and max speed is reached the vehicle starts to decelerate and now it is powered both by the vehicle kinetic energy and propeller kinetic energy that at this max speed point are at max charge and as they get used up the vehicle gets to the point where is again below wind speed.