He's a hakiman like Roger.
He is a former rival of the Mihawk, who disputed sword duels in the past. Then he lost his main arm (he was left-handed) and then he became much weaker than Mihawk and no longer competes for the WSS title.
So, to get around this huge loss of power, he received a huge power-up that put him on the yonkou level 4 years later. He got a technique for channeling the king's haki, which would basically be channeling his haki power with his sword skills.
So he stopped being a swordsman who relies on his cutting skills and swordsman techniques, and as soon as he lost his arm he uses his sword as an extension of his haki power, just as the WB's bisent is a weapon that extends the power of earthquake, and just like Roger's sword was an extension of the haki's power, or Kizaru's sword is an extension of his light powers. But none of them use their sword as their primary force to cut icebergs and compete for the strongest swordsman.
Shanks and Roger are traditional pirate prototypes, they don't even use a samurai sword, they use pirate sabers, traditional pirate weapons and Oda infacts that.
Shanks has never been said to be a swordsman, in any databook, in any manga statement, or in any magazine. Oda on SBS asked how Shanks really fights. It is very obvious that he is no longer a swordsman who disputes the WSS title