Since his quote came from a portion of one of my posts I guess it is up to me to explain.
1. I use slatted racks, made of round 5/8 or 3/4 dowling, that are spaced 3/8 inch apart. I do not use a screen of any type.
2. Overflow bees, that would normally beard around the entrance, take up residence on the slats and act as guard bees. They will sometimes festoon from the slats if a very large population, and hang down below the hive like a swarm.
3. Other bees, including wasps and hornets, seem to be confused around bottomless hives.
4. My tops are actually inverted bottom boards to which I add a entrance reducer to make the vent 3/8 by 3-4 inches, there are guard bees here, sometimes they use the upper entrance when the front and back entrance areas are exceptionally crowded. And some bees just opt to use the top entrance/vent as their only access/egress.