That's nonsense, Mages can beat Locks in PVE in T4, T5 and T6, Mage only really falls behind in near BIS T6 gear levels and in Sunwell. Also, Rogues from T5+ gear can beat locks on dps on single target, and Hunters can do so in every tier. Why do you think Melee has a hard time in PVE? A lot of Sunwell strategies were built around melee dps teams because of their supreme synergy and Sunwell is miles above the other raids in difficulty.
I played a Fury Warrior, Combat Rogue and Enhance Shaman in TBC retail, we did not have it tough... Dungeons were tough maybe for the Warr/Shaman, but not as the Rogue, they bring CC, strong dps and have threat management tools.
Warlock is supremely strong in PVP and a requirment for 25m raiding in every tier, but god damn are they boring in PVE when the only offense spell you use on single target is Shadow Bolt.