First of all, the one who actually stopped it only stopped it for a second and would have died if Law wasn't there to save him, hence Big Mom's comment on how "they escaped / avoided" it, yet the fraction of time he blocked the attack was enough to turn him into a mummy
Second of all, Kaido was capable to match Big Mom's napoleon attack while in base, so in hybrid form, he might be capable to generate stronger hits than her, and since hybrid, Kaido showed multiple other forms in his drunk form, with Luffy also commenting on his haki getting stronger, all of that suggesting that Kaido is capable to generate stronger attacks than whatever his contribution was in the hakai showing. And Luffy was capable to match that Kaido in all those forms even before G5, whose power is mentioned only to be limited by imagination
Third of all, i never said it would be strongest attack for sure, hence why i said it could depend on the scale and power of the attack(we've yet to have the visuals of the attack)
Not just that. I think this quote is also in preparation for his battle against Blackbeard who will have awakening of all his devil fruits, but Luffy ultimately beating him by showing stronger will power