That seems obvious enough, as witness any number of meetings I have attended.
But you have to look beyond the headline. The description in the story doesn't talk about meetings as such. Read Montegue, the lead author, was quoted as deprecating meetings but the rest of the details didn't match.
I got suspicious, and looked up the original paper in the Royal Society Bulletin. What they really studied was what happens when a small group is told your IQ. People did better on the tests when they worked alone than when they were in a group with everybody knowing everything at once.
So people get worse at IQ tests if they think that other people are judging them along the way. Not a huge surprise, but nothing like the headline. (BTW, truncated graphs like Figure 2(c) aren't a very honest way of expressing the data; they make the differences look larger than they are.)