So why was Whitebeard called the World's Strongest Man around Roger?
So why was Roger the one to amaze Oden the most, the one used as standard of strength to hype Whitebeard (not the other way around), the one with the title explicitly addressed as that which puts you above other conquerors?
Whitebeard was called the World's Strongest Man before and after the Great Age of Pirates if we believe Vivre Cards; in that interim in which Roger made his final move he established himself as the gold medalist, the top pirate in the world and the top conqueror and the standard for a person who knew the powers of both to hype Whitebeard's strength. Oh, and because not being the strongest forever nor a myth but a man was Whitebeard's whole narrative in Marineford, but this may be hard for power scalers to understand since it involves something beyond cartoon characters punching each other.
But sure, sure, the Pirate King wasn't the strongest pirate just because of one title that has been nuanced for years because Oda can't solve the contradiction with Kaido's own status as World's Strongest Creature (maybe titles are ultimately irrelevant...?).