Brainstorming solo versus brainstorming in a group are very different activities. Your assertion was that brainstorming with an LLM, which feeds back more-or-less like another participant, was the same as brainstorming with a rubber duck, i.e. solo, which it is not.

Brainstorming solo is an exercise in fully exploring your understanding of a subject, and developing new insights or approaches to it. Brainstorming as a group is more about finding a consensus solution or approach. Doing so with an LLM is much closer to the latter, however your "partner" has no actual experience but still presents feedback as though it did, and, critically, has an inbuilt tendancy to confirm your biases. Thinking of that like either solo or group brainstorming will automatically warp your thought processes by encouraging you to stay on certain tracks.