I said its a Gimick, basicaly its much harder to damage him with his flame on, not impposible same with Marco, people said it says he can regenerate from everyting it was also stated by multiple characters, now we know it has limits, just like King, and Zoro was doubting himself at that point, thats why he said it, i just looked, his spirit was down and he was feeling sorry himself, than he let go of fear of failure and he won easily. Simple just read the story.
Zoro injuring King pre ADVCOC with his flame ON