The flames were just overflowing COA.
The green flames appeared during the King fight without any Haoshoku when Enma went berserk:
And when the fodders fainted, there was black lightning above their heads.
We also see the same aura when Zoro first received Enma:
The flames did not first appear when Zoro unlocked Haoshoku coating. That's literally just memory holing Zoro's Enma fights.
Flame aura on Zoro's swords have never been Haoshoku, and it's kinda crazy that people invented this headcanon right now.
No one before today insinuated that Zoro had Haoshoku coating whenever his swords were surrounded in a flame aura.
The black lightning has been a reliable indicator for Zoro.
@sanjikun,
@Roo.