If GPT4 were given this whole thread as an input, it might reasonably be expected to predict a message that is a reply to a particular comment in the thread and presents a relevant and coherent argument based on the priors and "personality" encoded somewhere in its huge parameter space.

Sure, LLMs are not inherently goal-oriented. But that doesn't mean they fundamentally cannot exhibit goal-oriented behavior if either prompted or trained to do so. And if we remove the "one-off" restriction and allow the retention of some sort of an efficiently encoded state between invocations, who knows what might happen?