I feel like what old man Hyogoro said is more important than we think and what Luffy has learnt, Advanced Armament Haki, is also very important.
Luffy wanted to learn Ryuo to be able to hurt Kaido. With this technique, the user can project their Haki flow from their body without a medium. This method allows the user to damage and/or repel their target without making direct contact or using brute force.
However Hyogoro said Luffy that there is a higher grade of the emission technique. The emitted Haki flows inside a target and destroys it from the inside-out.
Now what if all the Scabbards have Ryou but not Advanced ryou which destroys from inside out?
This would make sense as they are able to cut Kaido’s scales and hurt him but haven’t been able to give him critical hits as Kaido has said so far in chappter 993.
This would mean the reason Kaido couldn’t heal from Oden’s slash is because Oden was just a master swordsman able to destroy Kaido from inside out.
That’s why Luffy needs Advanced Ryuo to be able to do critical damage to Kaido.