What they mean by "change the damage calculations of tanks"?

[Xenosan]

but I don't agree here at all. You forget about how arbitrary issues of DPS really are in balance. It's about ratios rather than absolute magnitudes.

I hate embellishing my worst-case scenario fear here, but the mention of ratios actually reinforces another reason why they'd have a bilateral nerf to tank DPS. The higher the DPS contribution of tanks, the greater the significance of the DPS imbalance between the 3 tank jobs (specifically that difference's relevance to the rest of the raid). If all tanks suffered the same DPS nerf via damage calculation, then their max DPS potential relative to each other remains the same, but the ratio does change for rest of the raid.

Hypothetical 50% DPS nerf:
WAR 900, PLD 700, Bard 1,000
Post scaling adjustment
WAR 450, PLD 350, Bard 1,000

The Ratio between WAR/PLD DPS is the same, but only half of what it was in relation to that Bard. When you cut down the DPS of all 3 tanks via the same means (damage calculation) you are subsequently cutting down the disparity between tank job DPS (in relation to the rest of the party) by that same amount. The difference is less significant to rest of the raid. Alternatively, increasing tank DPS unilaterally via damage calculation widens the gap further.

/Devil's Advocate.