Who says that this is a swordstechnique? Who says that this is all Shanks and Roger do?
lmao
Exactly.
It was used with both Axe and Sword.
A single sword technique does not make someone a swordsman.
Show me evidence that unicorns do not exist.
Its impossible to prove absence.
The way a character fights decides whether or not they are a swordsmen.
Shanks has so far not shown anything that would suggest that he is NOT a swordsman.
That does not mean that he can not show something in the future that could change this.
As it stands the most reasonable take is that Shanks is a swordsman and so was Roger.
NOTE: There are people who believe that Rayleigh may have been the previous WSS. This would mean that he was stronger than Roger. If he only became WSS after Roger died than Roger would have been WSS.
Everything Shanks does support he's a swordsman. There are multiple sources telling us Shanks is a swordsman.
The truth is, if Mihawk never existed nobody would question whether Shanks is a swordsman. The only reason they do is because people don't want Mihawk to be stronger than Shanks.
Your unicorn argument doesn't work here because that's real life and this is fictional material. In One Piece Oda told us Mihawk is the WSS and he obtained his title by traveling the world and fighting strong opponents until there was no more worthy challengers.
That's the information we have to go by and it's facts. Just because Oda hasn't revealed all of his past or showed us those battles doesn't mean that information is wrong or even Oda is wrong. The manga is not over so Oda has time to reveal more of his past and his opponents.
There's nothing in the manga or any official One Piece sources that state or even hints at Rayleigh or Roger was the previous WSS. We don't have any information stayed in Mihawk had to beat the former WSS to become the current WSS. Even if they were called the WSS at one time in the manga that is still different than Mihawk's situation. Oda told us Mihawk is the strongest in name and reality. That statement makes it very clear Mihawk truly is the strongest and it's not just what people believe. As long as he has the title there is no known or unknown swordsman stronger than him.