Many titles like the strongest swordsman, the strongest man, the strongest creature in One Piece are merely titles, given by people for their achievements and because they are very well-known.
Imu fits all three descriptions, yet few are aware of his existence. Within the World Government, only the Holy Knights and the Five Elders know of him; outside of that circle, only Shanks and a select few are privy to this information. Even the Revolutionary Army—whose goal is to overthrow the World Government—was unaware of a King occupying the Empty Throne; that speaks volumes.
For instance, figures like Mihawk, Whitebeard, and Kaido could travel the world and be instantly recognized across various regions. In contrast, if Imu were to walk the world, no one would have the slightest clue that he is the King of the World.
Imu is the strongest man, the strongest creature, and the strongest swordsman in the world—and he is far more than just those titles.
According to:
Rocks: He is the World
Shanks: His power is immeasurable
Gaban: He is God