I don't know how to further explain that a person's external facial and bodily responses are literally their external state and not their internal state. Your
interpretation of those is not actual knowledge of their internal state. To be even more plain, it's a
guess based on
external state. I know this is something that allistic people have trouble understanding, though.
You're mistaking the shadow on the cave wall for the thing itself. Inference is not the same as observation.
For the same reason that polygraphs are not lie detectors, the body language that you're convinced is "leaking" is not actually internal state.