"But 'Pirate King' is just a title!", many readers believe. And, to some extent, they're right: "Pirate King", as the title given by media, bears no meaning —but the way the piracy world conceives this notion is way, way more significant. But let's read Chinjao's words first:
This veteran explains it perfectly: the Pirate King is the supreme king who stands above the rest of the supreme kings, in other words: the conqueror atop conquerors. Not only does this reminds of Kaidou's portrayal of Roger of a supreme conqueror as prime example of the power that transcends all, but allows us to make some further connections regarding Roger's strength...
First, if Roger, per his status as king of all pirates, was the conqueror atop conquerors, then he had the strongest conqueror's haki; remember the whole conversation between Luffy and Chinjao revolved around having conqueror's haki and asking Luffy what kind of king he wanted to become. And guess what? The strength of your conqueror's can't be trained: it grows the stronger you become according to Rayleigh.
Which, if we follow some basic logic, leads us to an also basic conclusion: if Roger's conqueror's was above anyone else's, and if conqueror's can't be trained but grows stronger the stronger its user grows, then Roger grew stronger than anyone else...
...which isn't surprising because Luffy, the main character, the man who carries Roger's will, the man who wants to become the Pirate King just like Roger did, will also become stronger than anyone else:
But why does Luffy want to become the strongest person in the whole world? Well, as he well stated against Blueno, he wants to be the strongest so he can protect his friends. This comes from his childhood trauma of losing Sabo, as he felt he wasn't strong enough to prevent such loss:
But why is this so important? Because not only was Sabo's loss related to Ace and Luffy's weakness, but to the strength of the oppressors who killed him. The narrative here, as expressed through Ace, was that Sabo "died" because someone who was the opposite of freedom stole from him the chance to grasp freedom himself:
In other words, Luffy wanted to become the strongest in the world because he wanted to protect his loved ones from those who oppose freedom, so everybody, not only himself, would be capable of living free. This direct, explicit link between strength and freedom is foundational for Luffy's dream to become the Pirate King.
Because, remember, why does Luffy want to be come the Pirate King...?
And since Luffy wants to become the Pirate King because that's the freest person in the whole world, and considering Luffy links freedom to strength as he wants to become the strongest person so nobody who opposes freedom can do to his friends what they did to Sabo, what does all of this says about Roger's own strength?
Yeah, you know as well as I do.