Well, the thing is I'm not sure if Asura is basically a manifistation of Supreme king's haki, or did Zoro simply just tapped into the king's haki AFTER Asura was activated and he simply coated his attack with supreme king's haki by luck... and that Asura generally doesn't have supreme king's haki in a normal state...
And I don't want someone to convince me of either point cause I need future chapters to explain this to me Lol
Anyways, IFFF Asura normally doesn't have supreme king's haki, then it's a mode, and he can use many techniques, but if Asura is a manifistation of supreme king's haki, then It's just haki technique not a mode