If you look closely and objectively without any certain agenda in mind, you can see Zoro’s blades not 1 but 3, are covered in smoke and lightning streaks with long trails. When you see something like that, it is without a doubt Advanced Conqueror’s Haki.
Zoro in his flashback learned how to tame each sword (which has different personality) and bend it to his will. Zoro has tamed Wado Ichimonji and Sandai Kitetsu, so now only Enma left. The problem is, Enma this whole time is not 100% trusting Zoro as its master, when Zoro started to doubt himself, it started to test Zoro, Enma wanted to know if Zoro is really up to its standard.
How Zoro can bend such a temperament and having high standard blade to his will? The answer is by going all out without restriction with no fear of death and at the same time also accepting himself as a King of swordsmen that will force his blades to bend them to his will.
By accepting that he’s a indeed a King and giving all or nothing mentality(sunacchi), Zoro was pouring his Conqueror’s Haki to coat his blades instead of the usual Advanced Armament Haki. Zoro is Conquering his blades and bend them to his will in the process. That’s how he unlocked the Advanced Conqueror’s Haki.