It would be good to list a few possible ways of interpreting 'understanding of code'. It could possibly include: 1) Type inference for the result 2) nullability 3) runtime asymptotics 4) What the code does
Nobody interrogates each other's internal states when judging whether someone understands a topic. All we can judge it based on are the words they produce or the actions they take in response to a situation.
The way that systems or people arrive at a response is sort of an implementation detail that isn't that important when judging whether a system does or doesn't understand something. Some people understand a topic on an intuitive, almost unthinking level, and other people need to carefully reason about it, but they both demonstrate understanding by how they respond to questions about it in the exact same way.
Judging someone's external "involuntary cues" is not interrogating their internal state. It is, as you said, judging their response (a synonym for "answer") - and that judgment is also highly imperfect.
(It's worth noting that focusing so much on the someone's body language and tone that you ignore the actual words they said is a communication issue associated with not being on the spectrum, or being too allistic)
"Nobody interrogates each other's internal states when judging whether someone understands a topic. All we can judge it based on are the words they produce or the actions they take in response to a situation."
I'm fairly sure I wrote something that contradicts these two sentences.
You can commonly figure out whether people are leaking information about how they feel and what emotional and cognitive states they are in, even in text based communication. To some extent you can also decide the internal state of another person, and it's often easier to do with text than in direct communication, i.e. two bodies in the same space.
This idea that people aren't malleable and possible to interrogate without them noticing would be very surprising to anyone making a living from advertising.
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.
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.