First: Undocumented? So what? Either it can understand the code, or it can't. (And it better not depend on the comments, same as a programmer shouldn't, because they can be lies.)

You think it's unreasonable to expect it to understand 21 lines? Of go, one of the least-dense languages out there?

I think it actually demonstrates the point: (some) LLMs really cannot read and understand code if it doesn't fit in with their training data. They don't know the language, they know the patterns they have seen. If it doesn't fit, they can't reason about it from a knowledge of the language syntax, because they don't actually have such knowledge.