That's silly. My car is not absolutely guaranteed to turn left when I turn the steering wheel left, but you wouldn't say I can't control my car on that basis.
Steering an LLM with a prompt is way less reliable than steering a car with a steering wheel, but there's still control. It's just not absolute.
if your car doesn' turn left when you turn the steering wheel left, the problem is that the car is broken, if an LLM does something unexpected after you gave it instructions, that's possible when the LLM is functioning entirely correctly.