May want to reread this part.
He is talking CONTENT here, not class balance.
He is talking how the fights work, not how classes work in the fight.
What he is saying is they didn’t design fights to have 6 tanks needed then a 2 tanks needed then a 4 tanks needed for different fights.
They didn’t design the content around Dual Spec. They did design the classes though with the bring the player not the class mentality.