Pirate King Title
Roger was the strongest pirate. Laugh Tale is at the end of the Log Poses, and to go through that journey he had to overcome every other pirate crew. Calling him World's Strongest Pirate would muddy the title of Pirate King, it's redundant.
World's Strongest Man Title
Whitebeard was literally the strongest man. His fruit doesn't matter, his actual physical strength/will does. When he was alive, he was the biggest human man in the story. Kaido actually doesn't count as a "man" because of his biology, and Big Mom is a woman.
World's Strongest Creature Title
In Whole Cake Island, Big Mom referred to Kaido as "that thing". She wasn't being "colorful", she was referring to the Azure Dragon fruit properties turning him into a King of Beasts. In this case, the fruit did matter because it overwrote Kaido's biology.
World's Strongest Swordsman Title
This title refers to people who have actually caused Swordsmanship to evolve throughout different eras. Ryuma uses a one-hand sword style, Mihawk uses a two-hand sword style, and Zoro uses a three-hand sword style (he uses his mouth as a hand). At each point, these guys were the pinnacle of swordsmanship for their time. Well... Zoro hasn't achieved it yet, but still.
Notes
1. These titles were meant to show a gap between Roger's era and Luffy's era. In Roger's era, everyone was using modern titles that refer to a hierarchy below Godhood. The influence of that era peaked at the discovery of Imu who transcended the realm of mortals and had become a God (according to the Harley Scroll).
2. Kaido's title doesn't include humans among the "creatures". It's independent of Whitebeard's title. However, Mihawk's title of Strongest Swordsman is actually broad because a Swordsman can be something other than a human man. The issue there is that a Swordsman isn't like Kaido or Whitebeard, they're not at the top of a hierarchy that includes a domain like creatures or men. Instead, it's more specific. It involves men from any culture (including giants, long-arm, etc) who practice the Swordsman discipline, but because they're never ruling a domain, they're also not developing Conqueror Haki well.