A flat has one or two doors. How are you going to get "those large numbers" through one or two doors? You simply cannot "congregate, move about or proceed in large numbers" in that kind of environment, i.e. through one or two normal doors, particularly if there are passageways and stairwells involved.
As I said above: "If you're ever seen a swarm of bees, you will understand."
Dictionaries are wonderful things, but they do not and cannot mindlessly trump the context.