I doubt that an LLM for CAD would be using the free-form generation that seems to lead to the 5-legged chair.

I suspect it will be more of a tool calling layer that is building up the model based on the output of more deterministic tools. "I need this bearing mounted here" -> LLM reads the part number of the bearing, does a RAG search for the bearing properties to get sizes, looks up what type of fits should be used in the scenario, feeds that info into a function that generates the appropriate seat geometry. Rather than trying to morph triangles into sort-of-kind-of a cylindrical face that may or may have the 0.0XXmm interference that is the difference between success or failure of the fitment.