'Crowding' in this situation just means keeping more fish than would normally be recommended for a certain space/tank size.
It's more or less the same kind of thing that's done with some nippy fish, like tiger barbs, where you have to keep a certain amount to share out the aggression from the most dominant fish.
It's rountinely done with Lake Malawi and Tanganyikan cichlids, and sometimes with discus and angels.