Your argument doesn't really add up. A large and established guild must limit its recruitment, and taking steps to do so actually demonstrates experienced and effective guild management. Suggesting that guild management is "inexperienced" just because you don't like one particular recruitment requirement isn't a valid position to take. You have no inside information to support this statement and your sweeping comment that this is "the biggest mistake that guilds can make" is overly simplistic and naive.
I understand frustration with players that require unrealistic levels of gear before joining their heroic Hogger raid, but I personally believe that setting a (low) level of required gear is a useful additional tool to wean out people that aren't really interested, capable, or dedicated enough. It's not the only requirement, that's quite clear, and it's not so impossible to imagine that "good and friendly" members could have also run more than one heroic dungeon on this server before joining the guild.
The main point being that the requirement is a very gentle one. Despite being an end-game raiding guild, they're not asking for members to have the best gear available. Experience and "knowledge of the game" is expected though.