Maybe because if we use a clash of swords, Mihawk will win. Who knows. All Emperors are called the strongest pirates.
While Shanks story after he lost his arm was fighting pirates, doing pirate things, using his overwhelming haki and other stuff he was considered an emperor.
What about Mihawk? Well he was against the navy, showing how proficient he was with a sword, fighting some know pirates who could use a sword he lived for that, fighting swordsman and the Navy might have then known he used to duel with Shank, the Emperor and so then he might have been called the best swordsman.