Same as with Kaku, ashura is not just a attack, he can use it as form to block also other attacks.
Did find this comment from Zorofanboy video and agree with that comment:
Since Zoro not unlock his basic CoC he can just use ashura for very shorttime, I assume if he unlock his CoC and get stronger, I can see he can even use it as form as Luffy using boundman.
Seeing Kaido hyping Zoro CoC, im very sure Zoro use adv CoC iin form of manifest power in Ashura.^^