Yes, because he needed Jozu to protect him.
You have a fully understand the scene. Mihawk wasn't there to kill WB nor did he care whether Ace lived or died. Unlike the admirals who were actually trying to kill WB, Mihawk was testing WB because he was the first one to notice WB grew weaker. That was the purpose to his statement prior to attacking.
Mihawk saw WB need protection from his attack. Oda didn't just have anybody protect him. He had the crew member with the strongest defense protect him from a nameless hakiless attack. Jozu can turn his body into diamond one in a harder substance in the manga. Just because he might be the second or third strongest member of the crew doesn't make his diamond defense any weaker.