The author himself disrespects yonko title.
Buggy? Yonko
Goofy? Not taken seriously by anyone, not even an underling of the captain of his own fleet.
Blackbeard? a bitch
Meme? Speaks for herself
Kaido? jobber who lost to a rookie who trained while fighting him
The yonko "respect" only comes from WB and Shanks.
WB was already a legend prior Yonko shit and Shanks is just new Roger, if there was a higher title than yonko in piracy world, they would've already thrown measly yonko title long ago.
Oda basically tells us through Wano citizens that the feat of defeating kaido is something Ryuma is capable of.
No he doesn't. He just compares heroism.
What Ryuma did was just slay a fodder dragon. Punk Hazard Zoro lvl feet.
But it's impressive to fodder so they thought it was something amazing of a feat. They aren't powerscaling.
Ryuma is also a far bigger legend than Oden, who is close to kaido level.
How is Oden supposed to be a legend? He's still alive in the memory of most citizens of Wano.
Legends are distant things that are basically exaggerated stuff because no one seen how things went.