The Marines could be wrong about Kidd having a fruit (when Luffy met Franky and he breathed fire Luffy and Nami thought for a while that he had a DF). Like I said, he could be a magnet-controlling robot from the moon. I am not saying this is true, but if you want conformation for Buggy's Shichibukai status, it wouldn't be fair to accept Kidd's Devil Fruit without confirmation. The same things I said about Kidd also apply for Capone, Apoo, Bonney etc.
With the same logic we could say that Blackbeard could be wrong about his fruit being Logia. Even Rayleigh could be wrong too about haki. So do you think they could be wrong?
Nami and Luffy didn't know anything about Franky and his ability. But Blackbeard is well-versed in his DF, so does Rayleigh with haki. And I think Marine should be well aware of the ability of the Supernova rookie with the highest bounty, so when they say Kidd is an ability user, it pretty much confirmed it.
I agree that things are only fact if Oda says so, but I believe he likes to tell us through the people in the story. So unless Oda says otherwise (whether personally or shown later in the story), it's safe to assume that what people say in the story is the fact.