I wouldn't be that surprised if Paradise Totsuka could two-shot Kaido, or at least leave him vulnerable enough for a subsequent beheading. Swordsmen have always had ridiculous AP/lethality, and this was the strongest attack of a top-tier swordsman with ACoC landing clean on an enemy.
Not to mention it literally happened again - after the Scabbards used Paradise Totsuka, Kaido was laying on the ground for a bit, got up disappointed and as they were running to severe his neck said "I could have let you kill me but you people are not Oden". Kaido implied that Oden in the same situation could've done the same shit - he could've cut through him with Paradise Totsuka, knocked him down and cut off his neck. And Kaido would've let him as payment for what should've happened 20 years ago. Why else would Kaido say "I could've let you kill me but you people are not Oden" if Oden were not capable of doing so if he were there?