“Face–to–face” here covers any type of video conferencing, telephone calls, or even text chat that is happening in real time. As you said, these are synchronous interactions instead of asynchronous interactions.

> And it overlooks the fact that people might build on top of each other,

In the research that I read, the researchers came to the conclusion that most of the meeting time went to building consensus instead of actually thinking of new ideas. Some subset of the people in the meeting would be charismatic enough to dominate the discussion, and everyone else would contribute less than they otherwise would have.