I think he just meant being able to use armor set and it being able to AFK for 20 minutes. When using the Obsidian set (boxed in yellow) there is bonus damage applied.
From the wiki on the NMZ Strategies, "Using Dharok the Wretched's equipment is optimal with 92+ Hitpoints, without that the best is a toktz-xil-ak, obsidian armour, and a Berserker necklace. Otherwise use the best available strength bonus gear or Void Knight equipment."
Regarding the Obsidian set, "When used in conjunction with the obsidian platebody and the platelegs, all obsidian weaponry will be given a 10% boost in melee accuracy and melee strength. The damage bonus does stack with the Berserker necklace, multiplicatively, and is applied first. "