You don't get it.
Swords alone didn't allow him to cut Kaido's body. He needed powerful Ryu/haki. If haki is not strong enough or don't have enough AP the attack will be too shallow. Similar to the Scabbards combo attack.
That sky split is more about will power than AP. That's why Luffy still hasn't damaged Kaido with none internal destruction haki attacks.
Swords alone didn't allow him to cut Kaido's body. He needed powerful Ryu/haki. If haki is not strong enough or don't have enough AP the attack will be too shallow. Similar to the Scabbards combo attack.
That sky split is more about will power than AP. That's why Luffy still hasn't damaged Kaido with none internal destruction haki attacks.