The way I see it, emission and coating a blade/object in Haki uses the same principle. They are both flowing Haki in a certain way. That's why Hyogoro was known as the ''mighty blade'' in the past because he was capable of doing this. This is why it's called ''Flowing Cherry blossoms'', because in Wano, Haki is most used in this kind of way because of their way with the sword.
The difference is that with emission, you are flowing Haki without a medium, which forms the barrier around your body part. This is why I feel that you can't form a similar barrier with CoA on a bladed weapon already hardened with Haki, because you are already applying the same principle. That barrier from ''emission'' would already be used to coat the blade in the first place.
This is also how I see the relationship with Luffy and Zoro's power ups in this arc. In my mind, Zoro is likely extremely skilled in flowing his Haki, which is why he's able to survive Enma. He could probably use Emission if he wanted to already. However, he needs to learn how to use a large amount of his Haki onto the blade without the draining effect, which is what makes Enma a training tool. Luffy on the other hand, is already capable of using an extremely large amount of Haki with forms such as Gear 4, but his training involved learning how to flow his Haki like Zoro does. They are both learning what the other lacked in specialty.
This is just my interpretation of how it works based on what I see from the story. Maybe I'm wrong, maybe I'm right.