Perhaps it would help to remember that aircraft propulsion systems are not "lossless". Not all the power goes into moving the aircraft so you can't simply say the power generated by the engine is
Power = force x velocity
You have to write something like...
Power = (force x velocity) + "losses"
You can define:
force = thrust
velocity = aircraft airspeed
but best leave the definition of "losses" vague for the moment as there are too many things to list and they are NOT constant. Some of the losses are also dependant on velocity.
Now it's clear from the modified equation that the power output is not necessarily zero when the aircraft is stationary. In such a case "Losses" includes throwing a lot of air backwards for no purpose.
In the case of the car on the dyno, "losses" include the load the dyno places on the wheels (eg heating up the brake unit).
Interesting to consider what happens at take off. Lets say you have brakes on, run up the engines and then release the brakes... as the aircraft starts moving through the air the sum of all the "losses" must reduce to balance the equation. (I'll ignore the fact that some engines produce more power when moving).