i am not talking about the green smoke...
We see when enma uses massive amounts of ryou its smoke coming off zoro blade.
When zoro chooses to go all out with his haki we see excess smoke from all three blades and black lightning signifying advCoc haki
When he used bird dance there is no excess smoke coming off of zoro blade signifying that he is not using excess ryou here but as the black lightning shows he is flowing advcoc.
Bird dance was not a King Of Hell attack because zoro was not flowing both excessive amounts of advcoa and advcoc.