So much faltsetie here, Kaido never said he would've lost to Oden, ever. The one time he told Oden that if he decided to fight them right then and there and rally the entirety of the samurai and wano to his aid he said that the fight wouldve been difficult.
Oden was also going to Kaido's castle to try to kill him while he was Drunk/Asleep, and even went so far as to write that letter to Toki, that showed his uncertainty of actually fighting him 1vs1.
All in all i believe the two were relative to each other, but since Dragon form is ass Oden managed the first great hit on him. And if the battle went on then Kaido wouldve been at a disadvantage, but now we know he had a hybrid form too.
Oh and last but not least, show me the panels of Kaido having ''nightmares'' about Oden please.