no one's arguing that we can't (or shouldn't) have any criticism during brainstorming, only that negativity (which can also be read as aggression) reduces psychological safety, which in turn reduces creativity. people do pull back (consciously and subconsciously) when the sting of (negative) criticism looms.

idea-focused criticism (rather than people-focused) can hone decision-making, but we still need that psychological safety to get the best outcomes. these things can both be true at the same time, and we can both succeed and fail at fostering either or both and not get the best outcome.

just as we might criticize an overly safety-oriented decision-making process, criticism-oriented decision-making processes tend to degenerate into contests of aggression, which tend to produce inferior outcomes. dogmatism in any direction doesn't help.