What makes Roger not a fraud is that, at the end of the day, his peers recognized him as the king of pirates.
Chinjao described it quite explicitly: the pirate king is the conqueror atop conquerors. Which is why Kaidou recognized him as the person who conquered the world with the strongest power; which is why Shiki described the incoming era as Whitebeard's only after Roger died; which is why Oden saw something different in Roger that set him apart from even Whitebeard; which is why Whitebeard himself spread Roger's will upon his own death...
Compare it to Rocks, who has been treated as a fool by both Big Mom and Whitebeard and was completely inept at ruling his "crew": that's the difference. To this day, Roger is respected and recognized as the pinnacle of piracy, and didn't even need a fancy title for it.
Chinjao described it quite explicitly: the pirate king is the conqueror atop conquerors. Which is why Kaidou recognized him as the person who conquered the world with the strongest power; which is why Shiki described the incoming era as Whitebeard's only after Roger died; which is why Oden saw something different in Roger that set him apart from even Whitebeard; which is why Whitebeard himself spread Roger's will upon his own death...
Compare it to Rocks, who has been treated as a fool by both Big Mom and Whitebeard and was completely inept at ruling his "crew": that's the difference. To this day, Roger is respected and recognized as the pinnacle of piracy, and didn't even need a fancy title for it.