It's not that the other people have incomplete knowledge, it is that they disregard it.

From the article:

"If each team member is provided with all project statements, then the team can match the decision-making effectiveness of an individual (which is about 80%)."

So depending on who got which portion of which information is the critical part here, bringing communication and social skills into it. Human skills are then much more important then pure technical knowledge. This seems to be the truth in other settings as well.