So now that we know Zoro was holding himself back too much with Enma and overall, I think it is sorta symbolic with Zoro's role and development this arc.
"Holding himself back"
"sacrificing himself for Luffy and the group with Hakai",
"throwing himself in harms way to protect Luffy and his dreams"
Zoro spends so much time worrying about his crew, and worrying about his captain, and supporting Luffy. But the fact is,
he's not like Sanji or the rest of the crew.
He is like Luffy.......He is a Conqueror, he is a King, with his own ambitions to sit on top of this world.
His development this arc, is coming to full terms with that.
This panel is such a huge contrast from where he ended off last chapter.
We were hit with a red herring from Oda. The Shamisen sound was the red herring, we were made to believe that is why Enma drained Zoro's energy,
I think it's actually because he may have doubted himself for just a moment I think, when he said he feels if he does not find a way around King's durability (knowing his race/powers), he would not be able to defeat him.
That is why Enma reacts, testing Zoro to see if he can really meet standards.
When Zoro understands this, it's like a total 180 from where he left off in the last chapter.