Why is it a big deal? It's their epiteth, just like Sanji's is Black Leg, and Jinbei's is Knight of the Sea
For almost the whole rooftop he just knew Enma and Oden. Queen wasn't in the rooftop. Why would he think highly of a dude with a 320million berry who came out of the rooftop bandaged in a cross and teams up with "Judge's son" to fight?
Queen asked if Sanji was talking to "the Pirate Hunter", then he said "He can't beat King", just like he thought Jack wouldn't be defeated.
Big Mom, after knowing who Kaido's opponents were going to be, she said "You guys can't beat that thing"
It is simply a plot device to hype a character