I understand and I disagree. This has everything to do with dualistic thinking, because you and everyone in that room are thinking that way. The very idea of source/not-source is dualistic.
A lot comes down to our inability — our language’s inability — to talk about things emergent. Yes, one person in the brainstorming group articulated the vision, and that still came out of a process that involved several minds.
I understand that people get emotional about this once they are recognized as source and a lot of tension goes out of the room, and I think that all happens because you are resolving the dualistic tension caused when we misunderstand that ideas can be emergent in nature, so everyone in the situation needs a resolution to the (false) dynamic of having to attribute ownership to something.
I still think that in a brainstorming session the ideas are emergent, a product of the combination of minds, and ascribing ownership to one person limits the range of possibilities that could come later.