That's the wrong comparison. You want to compare structured brainstorming against all the people who would have been in the brainstorm working solo. Otherwise,

That's the wrong comparison. You want to compare structured brainstorming against all the people who would have been in the brainstorm working solo. Otherwise, all you're finding is that N people are more effective than 1 person. That'll often be true; the question is how to make the best use of those N people, and the claim being made here is that brainstorming is worse than having them work mostly independently.