Rumors, the same way we have had rumors of sun god Nika until its appearance. Maybe nobody approached the ship enough to know that it's Dragon. Maybe the only person who survived this event is someone who doesn't know Dragon or couldn't identify him. There can be multiple reasons.
Dragon's ship is also called "Wind Granma", how fitting. Guess what is Luffy's ship name? The Sunny.
It is very likely that Dragon's fruit is related to making wind and whirpools.
More interesting facts about Dragon's ship (= Wind Granma):
Granma is also the yacht that was used to transport 82 fighters of the Cuban Revolution from Mexico to Cuba in November 1956 for the purpose of overthrowing the regime of Fulgencio Batista.
Batista initially rose to power as part of the 1933 Revolt of the Sergeants, which overthrew the provisional government of Carlos Manuel de CΓ©spedes y Quesada. He then appointed himself chief of the armed forces, with the rank of colonel and
effectively controlled the five-member "pentarchy" that functioned as the collective head of state.
Batista seems like a parallel to Im and the five-member pentarchy to the Gorosei / Five Elders.
The wiki page then goes on to say: "The Pentarchy included a representative from each anti-Machado faction." This supports the theory that the Gorosei is made up of a representative of each anti-Ancient Kingdom nation.
Source: Reddit