the sword draws ryou on it's own and cuts deeper than intended. it's not so hard to understand. and ryou is different from basic hardening. it's simple logic if you pay attention:
→luffy couldn't damage kaido with basic haki
→he tries to recreate an ability he saw Rayleigh use hoping it'll make him damage kaido (an advanced ability makes you produce a punch powerful enough to push enemies away without touching them)
→turned out that ability is called ryou (at least what hyougoro understand from luffy's description)
→luffy trained to master ryou
→enma automatically draws the user's ryou
→red rock attack is imbued with ryou
→enma and red rock can damage kaido
Ryou and haki are the same. Hyogoro explained that to us. Citizens of Wano call haki, ryuo.
For a swordsman to be able to use haki with their sword they must be able to flow haki from their body and infuse their sword with it. If not, they can't use haki with their swordsmanship.
Understand, swordsman can still just release haki from the body and attack with it directly. They normally don't because that's not their fighting style.
What Hyogoro explained to Luffy is part of the Pinnacle of swordsmanship power Zoro's sensei told Zoro about when he said there are swordsman who could cut nothing. Then used the (same) example of swordsman who can cut steel and not break paper. Which proves Zoro had haki/ryuo pre TS. Zoro just didn't know that because sensei didn't explain that to him.
When Luffy was trying to gain the ability to release haki from his body he want one level beyond that and obtained the ability to penetrate targets internally with his haki and destroy it from within.
I explained haki.
Like I said, enma doesn't make you better at controlling haki. It just makes it harder for you to use haki because it wants to release more haki than the user whishes. Enma users can become more skilled at using haki by learning to gain more control over there haki. Not because enma instantly makes you better. Remember, wearing a weighted vest doesn't instantly make you a faster runner. You become faster because you become stronger from wearing the vest.
Enma doesn't allow you to hurt Kaido. It's powerful ryuo or haki that does. That's why all of the Scabbards able to hurt Kaido without ever using enma. Even if you use enma if you have weak ryuo you still won't be able to hurt Kaido. Oda hype up Zoro's haki several times. When he cut/killed the dragon, cut the mountain size golem and beat Pica "invincible" haki.