well hes not a jounin.
its never said anywhere that hes a jounin in the series - anime or manga.
a site can say all they want to it doesnt make it true, the only hint we have is that Zabuza says he cant beat that jutsu, he could be at jounin LEVEL but hes NOT, i repeat NOT a nin that belongs to any village so its useless for him to carry a ninja rank.

Ninja ranks are allocated to suit ninjas to their appropriate jobs. They're not just a show of strength

that site was made by FANS its <u>NOT</u> official.