Chinjao was very clear in his explanation. Roger, as a user of Conquerors Haki, stood above all other users of Conquerors Haki. Luffy using Conquerors Haki, is what prompted Chinjao to make the proclamation in the first place. It has absolutely nothing to do with what he achieved, and strictly based on Roger as a Conquerors Haki user standing at the Apex of all others.
You would only have a case to argue if Chinjao didn't mention the Admirals in there alongside the Yonkos. The Admirals don't go around conquering territories, and achieving great things like the Yonkos. Their titles are strictly based on their individual strength.
And Roger has clearly never defeated Kaido and Whitebeard. Heck, he ran away from Big Mom as well.
Roger stands above others because he conquered the seas, by going where Oden told him to go.
As long as you dont have proof of Roger beating Rocks, Kaido and Whitebeard in 1vs1, he didnt stand above everyone.
We have the exact opposite, we have proof that he couldnt stand above Whitebeard despite trying for 3 days.
And it is Whitebeard who is WSM, not Roger. It is Kaido who is WSC, not Roger. Roger simply aint what you think.
He is the greatest because he found the treasure not because he was stronger than all others.