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

[Ibi]

Or they just make subsequent tank busters hit harder? No one gets hit with tank busters or boss autoattacks aside from tanks so Devs have plenty of room to make them hit harder.

The problem with making tank busters hit harder is that it makes little-to-no difference to what the tank has to do. You're likely popping a cooldown regardless of whether it hits for 15k, 20k, or 25k (maybe a different cooldown depending the damage, but that's about the extent of the difference).

It's the healer's job that gets harder as tank busters hit harder.

[Rbstr]

The problem with making tank busters hit harder is that it makes little-to-no difference to what the tank has to do. You're likely popping a cooldown regardless of whether it hits for 15k, 20k, or 25k (maybe a different cooldown depending the damage, but that's about the extent of the difference).
It's the healer's job that gets harder as tank busters hit harder.

Right, it doesn't make a difference. That's the point: He's worried that the excess vit will trivialize tank busters. But that's just based on current numbers.
The counter point is that it's just a balance factor. All the numbers are simply designed to some ilevel and skill level with current HP levels and healer HP/s capabilities in mind. It's all just numbers and timers that devs can do nearly whatever they want with.
But, specifically to you, you can make busters hit harder (and thus threaten higher HP-ed tanks) without increasing the actual DPS put on the tank by manipulating the rate. The amount of HP/s you need and the actual magnitude of hits are not the same thing. You need the rate to determine how much healer stress it causes.
There's no healer stress if all a boss ever does is a 300k tank buster attack 30 seconds into the fight. The tank always dies....but there's really nothing for a healer to stress about.