I think the point is that while it does act as negative acceleration there isn't a causal relationship with the actual speed of the car, which is mainly related to how far the gas pedal is pressed.
Drag is a causitive input to speed though.
nextCarSpeed(currentSpeed, wheelPower, dragForce, mass, deltaTime) =
currentSpeed + ((wheelPower / currentSpeed - dragForce) / mass) * deltaTime
Bertrand Russell objected to the notion of causation in the 1900s, because merely stating the updated dynamics of a system doesn't imply causation in any grand sense. Like hume, he dismisses causation, but not because of the problem of induction or anything, but because the concept seems incoherent to him. He especially emphasized this in physics - although maybe you can argue that for everyday human language, causation is good (Alice caused BCD to happen), in physics it doesn't belong.
Not that I entirely agree with his account but just some food for thought.
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In this example you're determining the speed of the car based on the wind flow on your hand. Putting the book in front might slow down the car, and it will probably also slow down the flow on the hand. However, if you still try to determine the speed of the car from the air flow on the hand, you'll probably be off, because car speed and wind flow aren't linked like that when the book is in the picture.
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