The WSS might only have become a thing as the great era of piracy started and Mihawk actively started to challenge all the swordsmen until he came to the top.
But I too believe that Roger, rather than being a paragon of a swordsman, was just someone who embodied the pirate-theme. Like Shanks' crew. Just a pirate with a saber, occasionally a gun, just using it as a tool.
If Roger's main asset happens to be swordsmanship (cut everything philosophy, flying slashes) then he is very well a milestone for Zoro.
If Roger just uses the sword as a tool akin to Whitebeard using his Bisento and Kaido using his club, rather focusing on Haki blasts and barriers (WB vs. Roger clash) and has another main power which is even greater than his sword related abilities, like advanced CoC which serves as a counter to Whitebeard's main power (Gura Gura no Mi), then I don't think Zoro would consider him a milestone.
Just entertain the following thought: Take Whitebeard's Bisento (a supreme grade blade) and change it into a long sword (still a blade, just the hilt to blade ratio is different) and let him keep his fighting style; just swinging the sword as he swings the bisento to clash with Roger using a Haki barrier, also heavily relying on his main power (Gura Gura no Mi) which he uses to imbue both his fists and his blades with. Did he suddenly become a swordsman? I doubt Whitebeard would even be nerfed if you changed his bisento with a long sword of the same grade, since all what changed was the blade to hilt ratio.