If that was the real reason then there would be no single player guilds in platinum or diamond league.
How many of those guilds are in a top 10 spot in their respective worlds? In my world, most of the top guilds have 50 or more players. The smallest one has 14 and everyone of those 14 players has more than 7million points. Yes a one person guild can do alright In platinum or diamond but who stands a better chance - a guild with one good player or a guild with 14 or more good players?