First: true propositions (that are not provable) can definitely be expressed, if they couldn't, the incompleteness theorem would not be true ;-)
It would be interesting to know what the percentage of people is, who invoke the incompleteness theorem, and have no clue what it actually says.
Most people don't even know what a proof is, so that cannot be a hindrance on the path to AGI ...
Second: ANY world model that can be digitally represented would be subject to the same argument (if stated correctly), not only LLMs.
I knew someone would call me out on that. I used the wrong word; what I meant was "expressed in a way that would satisfy" which implies proof within the symbolic order being used. I don't claim to be a mathematician or philosopher.