Yes but if Kaido was a dragon, why wouldn’t he be a mythical dragon fruit user?
Because, as I explained, the Ryu Ryu family probably isn't meant to directly mean "Dragon Dragon family" but "Saurian Saurian family that happens to use the ryu particle in Japanese since it's a common denominator". An hypothetical example to explain this better: "hippopotamus" literally means "river horse", so imagine that we're reading Two Piece and the Hippo Hippo family is introduced, made of all kind of hippopotamus models. So we theorize and assume that horses will belong to it too because "hippo" means horse and then appears the Uma Uma: model horse. And we're like "How is that possible? Doesn't "hippo" mean literally horse? Then why isn't the actual horse part of that family?", while ignoring that the Hippo Hippo family would take its name not of its literal meaning but as a particle in "hippopotamus", just like "ryu" is a particle in dinosaur, pterosaur, lizard..., among others but that doesn't make them real dragons.
Want a more recent example? Imagine we have the Ushi Ushi fruit (Cow Cow) and the Hitsu Hitsu fruit (Sheep Sheep). The user of the muskox power appears, we assume it will belong to the Ushi Ushi family (because it's an ox by name) yet is included in the Hitsu Hitsu because that animal is actually a goat and not a bull.
I assume that's the train of thought Oda followed. Eastern dragons have more to do with fishes and water animales in general than with reptiles in many ways, but since they look saurian and they're associated to their Western cousin people assume they must be more related to a dinosaur than a fish but this isn't necessarily the case.
