He isn't actually.
1. Kanjuro didn't know Raizou was on Zou. He only suspected that was where he would be. Whereas Jack knew for a fact that Raizou was definetly on Zou and that the minks were hiding him.
2. The only way to find Zou is by having a mink onboard, or by having a vivre card of someone on Zou. If they had the vivre card of a mink on Zou, then that mink is most likely the traitor, as it would be hard to get a vivre card without that person's knowledge. Kanjuro wasn't on Zou at the time of Jack's first attack, so they couldn't have been using his vivre card, and he isn't a mink so he doesn't know how to get to Zou either.
3. Kanjuro knew about the ankle mark symbolism from before he even left Wano. He could've just snitched to Orochi and Kaido then and had all Kozuki allies captured then.
Orochi and Kaido only found out about the ankle tattoo
after its meaning was told to the straw hats, Law and Carrot.