Still the whole LLMs are based purely on pre-existing information and their magical way on how to access it. So e.g. you create (with the help of AI) a completely revolutionary product. Let’s say a new type of phone – so much different as iPhone was to everything before.
How will your assistant be of any help? There will be no experience with the product, and no one will ever write anything down in the forums. Not just about your product – about the whole category, that never existed before. The LLM would not know anything about it whatsoever.
Or let’s say Elon is successful and people get used living on Mars. And it would run Discourse-less (can you imagine 
?). Where will LLM get any answers about anything practical on Mars?
And there you go. It’s a combination. Without the knowledge base (forum) you just have an expert system but without the content.
So let’s say ChatGPT is able to help me with Discourse today. But how come? Well, because of Meta. Without it, LLM would just hallucinate.