Getting into the weeds, but I don't agree with this construal of what propositional content is or can be. (There is no single definition of "proposition" which has wide acceptance and specifies your condition here.) There is no similar way to assess truth outside of formalized mathematics, but the encoding of mathematical statements (think Gödel numbers) comes to mind; I don't think that the ability of the machine to understand propositions is necessary in order to make the propositions propositional; the system of ChatGPT is designed in order to return propositional content (albeit not ex nihilo, but according to the principles of its design) and this could be considered analogous to the encoding of arithmetical symbolic notation into an formally-described system. The difference is just that we happen to have a formal description of how some arithmetic systems operate, which we don't (and I would say can't) have for English. Mild throwback to my university days studying all of this!