1) Kaido isn't weakest yonko. This fight was 20 years ago , maybe Kaido didn't reach his full potential yet. Shanks became a yonko only a few years back while Kaido was already a Yonko and Kaido and shanks were both apprentice pirates under rocks and PK at start of their pirate careers.

2) Kaido needed to use a dirty trick . Oden was with PK and whitebeard ( he trained with both or at least learned from observing them) , so he is STRONG. Kaido hadn't reached his prime. So it was a close dif match and he is a pirate. You think he'll want to fight fair if it could result in him being defeated.

3) It was shown in manga that scabbards knew Izo would remain with WB when Oden returned. Did they not mention the condition of wano to him (Also they didn't tell him about Kaido )? If they did , I don't understand why didn't WB go there , since he knew Kaido is a powerful pirate and Oden's (his brother)homeland was under attack. Hell Izo , his son's homeland was under attack.