It doesn't matter that the matchups between Shanks and Mihawk were 13 years ago. What matters is that
the title is still valid today and is something accepted by everyone.
It means that even though Shanks is a Yonkou, they still consider Mihawk a better swordsman and to this is added that we know that Mihawk could also be a Yonko if he wanted to. And the difference should be little between Yonkos, so Mihawk >= Shanks is perfectly feasible.
Whitebeard's ship >>>>>>> Any rival of Shanks