Only the viewers, a few other characters directly involved in the events, and perhaps some of the most insightful and knowledgeable cast members, ever know the full truth of things. For all the world as a whole knows, Arlong was a really big scary motherfucker that often tried to outright murder Marines and that no one could bring down until Borsalino stepped in. And let's face it – Jinbei probably would've lost that fight too.

One Piece is very good with unreliable expositors and such things.

You don't have to look much further than Buggy to realize you shouldn't trust rumors in the One Piece world.