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.