Say 4 people are discussing things in two separate threads at the same time… by flattening that you essentially get two separate intertwined, but possibly unrelated, conversations. It’s going to be difficult to follow.
Put the same 4 people into a flat discussion from the start and they’re a lot more likely to address each other and stay closer to each other on subject matter, because they have no choice but to see everything that’s happening together. From my perspective, at that point you have an entirely different conversation.