I mainly agree with your argument, but I think "inevitable in hindsight" needs reconsideration. Following the incongruity model, humor comes when the punchline forces a change in perspective on the setup. So it is not that the new perspective is inevitable, but that it was such a less likely interpretation of the setup that it didn't occur to the audience.

Considered this way, the question is whether an LLM could be trained to create joke setups that are ambiguous in the right way, with one obvious interpretation and a hidden but possible interpretation that will be forced by the punchline.