It is said that to surpass Roger, you need to go through Admirals and even Yonkos; Roger did none of these things. That's why the statement uses the word
'surpass' and not
'equal' to Roger.
This means that if Luffy becomes the Pirate King, he will have already surpassed Roger and won't be on the same level as Roger but will be stronger than Roger.
But wait, logically, if Luffy becomes Pirate King, he should be equal to Roger. However, the word used by Don in Dressrosa is
'surpass,' indicating that becoming Pirate King in this era is different and more challenging than the previous era. SOMETHING suggests that Roger is not the strongest, as he has never been described as such.
Do you know what I think? It's that Roger isn't capable of defeating the Emperors and Admirals, and that's why someone who is above Roger's level is needed to do that. That someone is Luffy, who won't just match Roger's power when he becomes the Pirate King, but according to Don, Luffy will surpass Roger.
So, in summary: PK Roger < PK Luffy
Luffy doesn't need to defeat Imu-sama, the final villain, and bring down the World Government. The moment he becomes the Pirate King, he will be stronger than Roger and not on the same level. Roger is a fraud.
And to conclude, this implies that, in Don Chinjao's view, the accomplishment of defeating these powerful figures (Yonkou and admirals) would be an achievement surpassing what Roger achieved in his time. The challenges and adversaries in the current era are more difficult than those faced by Gol D. Roger in his time. If a person is able to overcome these challenges, they would be considered superior to Roger in terms of the difficulty of the achievements and in strength.
And you know what's funny? It's that Kaidou has been explicitly reiterated by the author as being a challenge greater than any Roger faced in his time, a challenge that only Joyboy, with a lot of help, managed to handle. 'But but but Roger > Kaidou,' narratively, that doesn't exist.
In the "One Piece" narrative, the challenges in Joyboy's era are conceived as being superior to those faced by Roger. This is suggested by the choice of the word "surpass" instead of "equal" when referring to Don Chinjao's achievement of the title of Pirate King.
In this context, your interpretation implies that Kaidou is considered superior to Prime Whitebeard (Whitebeard at his peak). This suggests that, within the story, Kaidou is presented as a more formidable challenge than even one of the most powerful pirates of the previous era, Whitebeard, who was equal to or stronger than PK Roger. It is stated that Roger surpassed Whitebeard as a pirate, not in terms of strength, and Oda does exactly what I just mentioned; he presents Kaidou as the World's Strongest Creature above Roger's greatest challenge, which was Whitebeard. If it's not Whitebeard, it would be Xebec. However, the current Joyboy shares something in common with Roger, namely, the need for assistance in dealing with Kaidou, and the other with Xebec.
