Ryou is not a technique, its a power statement.
Penetration Haki is a technique, it is manipulating Ryou in a certain way but not every form of Ryou would hurt Kaido.
You need certain level of Ryou to overpower Kaidos skin durability and wound them.
Luffy would never be able to do that hence why he needs a shortcut of just skipping that durability and targeting internal organs that are not very durable
Penetration Haki is a technique, it is manipulating Ryou in a certain way but not every form of Ryou would hurt Kaido.
You need certain level of Ryou to overpower Kaidos skin durability and wound them.
Luffy would never be able to do that hence why he needs a shortcut of just skipping that durability and targeting internal organs that are not very durable