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
Drag is a causitive input to speed though.
Increase "dragForce", and the resulting car speed decreases. That is a causal input, not an association.