Oda is not calling anyone as strongest. There is no such a thing Oda is informing the viewers from outside of the manga. He just said Kaido > Akainu in SBS one time, and when they also asked who is stronger Zoro or Sanji, he doesn't say anything, as well as he didn't say anything about who is stronger Akainu or Aokiji, or Kizaru in the SBS.
Every strongest / greatest title is just a rumor. There are no official titles and they are not like military ranks.
As well as mihawk's title is no different, it's just a rumor.
Zoro Novel: ''One marine claims that the strongest swordsman in the world is definitely Mihawk, the other marine claims it's Zoro. They witnessed their sword techniques in marineford and dressrosa. Mihawk cut down gigantic ice block. Zoro cut down gigantic Pica. They start talking about other candidates: Vista, Shanks and Ryuuma. A marine says Vista was able to fight Mihawk equally for a short moment. Ryuma is strong, but he is already dead. According to another marine, Shanks is said to be the rival of Mihawk. He faced Akainu without fear. 2 of the marines have heard that Shanks is a very strong swordsman. But another marine says it's not that Shanks showed his great technique with his sword in Marineford.''
--- Translation by Sandman
Anyone with a brain knows Yonko > Yonko commander, but they are comparing Shanks to Vista when it comes to greatest swordsman title.
Greatest swordsman title doesn't prove anything in terms of overal combat power, which includes Haki levels, durability, strength, speed, etc. etc. That's the reason people definitely believes Whitebeard or Kaido > world's greatest swordsman, there is not even a doubt that world's greatest swordsman can be equal with them. Even mihawk accepted his inferiority against Whitebeard because of this reason, mihawk's level when he achieved the top level swordsman, is still not enough to compete with Yonko level.
But Shanks is.
Shanks > world's greatest swordsman, Shanks is level of above of that useless title.