Kaido might get damaged with Ryuo, but it's still superficial and quickly recovered. The hand that got stabbed. Recovered within 2 panels. 4 Scabbards combining their attacks couldn't reopen Kaido's scar. A CoC infused Thunder Bagua from Hybrid Yamato gives him a small head bleeding. Getting pummeled the whole day with internal destruction CoC Gear stacking can't put him down. Meanwhile crows created from soot puts Fujitora in bandages for days.
Exactly, Ryou is an advanced state of Haki that aims to ignore the opponent's damn durability and injure them. However, Kaidou is so durable that even with Ryou, you can only superficially pierce him, causing only superficial damage that is completely resisted by Kaidou's incredible resistance and subsequently, it is a damage that can be recovered with his Zoan recovery.
Even attacks that penetrate deeply into Kaidou's flesh to the point of leaving a scar are being "healed" in minutes. Even attacks coated with COC can only cause small bleeding in Kaidou. It requires Acoa and Acoc combined to be truly effective. However, for admirals, even attacks without any basic Coa are damaging them (Fujitora). With basic Coa, you are damaging any Logia. Ryou and Acoc are exaggerations, they are not Kaidou.