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

[Cynric]

Dont forget traited Conva + Bulwark + Sheltron + Clemency + Shield (passive block).

PLD is the Physical Mitigation King.

Convalescence isn't physical mitigation. You could argue it enhances Shields like Adlo and Noct sect however that's general mitigation and also present on Dark Knight as well.

Sheltron blocks one attack and if you aren't the best at timing it or you experience latency, it can block an auto attack.

Clemency is healing and mostly interrupted, Not sure how this is mitigation.

Bulwark increases the block rate for a short time.

Dark dance increases the parry rate for a short time. Either way RNG is still RNG.

Passive block is RNG.