that's what I should ask you
why do you think this attack is their strongest even tho we haven't see them go all out
there's no need to damage control anything, this attack can kill Zolo anyway
Because Kaido is in his strongest mode and because both used this attack as a finisher, planning to end the battle with that in a single shot. Kaido also used another named attack in this chapter that base Luffy just tanked apparently. It's clear that the Hakai attack was on another level.
It's the strongest attack we've seen from both yet, so until one of them shows something greater, I gotta think that's the strongest.
Keep in mind that this is a combination attack, so it was greatly amplified. It's debatable whether Kaido or BM solo can dish out this level of DC.