When you start answering a question with question, more often than not, it means you don't have an answer for it and yes It really doesn't
- Could be that Zoro suspects that the Lunarian invulnerability isn't full proof and can actually be bypassed with AP over a certain threshold given King's behavior in their fight.
- Could simply be that Zoro doesn't believe that any one thing can be invincible given he doesn't really believe in higher beings like gods etc hence why he said "practically invincible" as opposed to simply saying "invincible".
Zoro has the perfect opportunity to actually prove your claim in this fight. If he doesn't it simply means he's unable to, it's as simple as that.