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.
I think most people have experience with situations where someone else has tried to hide their true feelings and still been able to figure it out due to involuntary cues. If you were to ever look at some popular television shows you'll find that this is a common basis for both drama and comedy and widely understood by people all over the world.
It might come as a surprise, but everything you've ever experienced is just shadows on the cave wall that is your cerebral cortex. You've never ever been outside of that cave and have always been at the mercy of a mammal brain preparing and projecting those shadows for you.
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.