The question is what do you mean by proximity? Is this only physical proximity?And does it mean that if you isolate people, but they are within 10 feet of each other they are more productive? And do the results change when there is not physical proximity, but substitutes or alternatives?
Open floor plan hotel seating with 1' distance between you and the next person who eats raw onions at their desk while talking to their spouse constantly
It's in the study, generally it's sitting close enough to converse
they didn't study alternatives to proximity, which isn't surprising because I'm not away of anyone that regularly works with a constant audio or video stream active?
It's probably pretty close to the Allen Curve.
https://en.wikipedia.org/wiki/Allen_curve