Buggy literally stated It.
Also, are you ignoring that to prove to be the strongest according to Oda you have to defeat Yonkos and Admirals those Who are strong in the world? Where Is Mihawk?
Also according to Ace's novel Shanks was One of the 4 strongest pirates in the world, and the novel isn't talking about military Power but the strength of the individual since It was questioning Who could Scar shanks, one of the strongest 4 pirates in the world (and yes, Mihawk being a warlord means he's a pirate).
I'll make a thread about this since I got tired of Reading Mihawk above Shanks or Mihawk top 1 when Oda debunked this.