I will try to refute you by understanding what you say.

I am not sure what you mean by "refute" here. Prove my conjecture wrong by giving a counterexample? Show that my arguments are wrong? Show that the examples I used to make my point clearer are bad examples? If it's the last one, but then I would not call it a refutation.

I guess that by 'meta-' you intend to say that at least some sentences in the meta-language couldn't in principle be translated into a non-meta 'human' language. Is that right?

Indeed, at least not without some extra layer of meaning not originally expressed in the language. To give another example (not a proof, just an illustration of my point), you can sort-of teach a parrot or an ape to recognize words, to count and maybe even to add, but I don't expect it to be possible to teach one to construct mathematical proofs or to understand what one even is. Even if a proof can be expressed as a finite string of symbols (a sentence in a language) a chimp is capable of distinguishing from another string. There is just too much meta there, with symbols standing for other symbols or numbers or concepts.

I agree that my PhD defense example is not a proof, but an illustration meant to show that humans quite often experience a disconnect between a language ans an underlying concept, which well might be out of reach, despite being expressed with familiar symbols, just like a chimp would in the above example.

What reason do you have for thinking an AGI's goals would be complex at all?

I simply follow the chain of goal complexity as it grows with the intelligence complexity, from protozoa to primate and on and note that I do not see a reason why it would stop growing just because we cannot imagine what else a super-intelligence would use for/instead of a goal system.