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