Because all titles are just rumors, it doesn't come directly from Oda. Info boxes were always deceiving and it's what people knows in One piece.
Mihawk is stronger than Vista? He needs to prove it. He didn't, he couldn't pass Vista and do what he wanted, he couldn't make Vista sweat. Even in that big mom vs Marco, we see Marco sweats. And Marco > vista. How can this mihawk guy can be equal to Yonko when Vista can easily stop him from what he wanted to do? He asks Vista's permission to end the duel. They are same level. If Vista can do that to mihawk, what Marco would do to Mihawk? Far worse.
Mihawk is stronger than Vista? He needs to prove it. He didn't, he couldn't pass Vista and do what he wanted, he couldn't make Vista sweat. Even in that big mom vs Marco, we see Marco sweats. And Marco > vista. How can this mihawk guy can be equal to Yonko when Vista can easily stop him from what he wanted to do? He asks Vista's permission to end the duel. They are same level. If Vista can do that to mihawk, what Marco would do to Mihawk? Far worse.