I take it you haven't really kept up with warlocking since 5.0? Warlocks may wear cloth armor but with Dark Apotheosis they're not exactly "clothies" any more. Notice how he was demo spec for bosses 2-5. It's not exactly hard to do bosses 2 expansions old with 20x the HP and 10x the DPS the average person had back then, along with tank damage reduction levels of 60-65%.