We actually don't know much about Ryuma's story. We know the fragment of him slaying a dragon ("Monsters" one-shot) and that he was known as the strongest swordsman of his time. As such, he probably left Wano and brought Shusui to fame... the "Monsters" story location, for example, doesn't look like Wano.
It's also unknown how old most of the meito are. We know only that Wado Ichimonji/Enma are newer than Kozaburo, their creator (100 years or less?), and Sandai Kitetsu/Ame no Habakiri are newer than Sukiyaki (he's 81 years old), but most others may be even older than the Void Century. That way, the knowledge about Wano's meito would be more accessible, as Wano was an open country in the past.