"Exuding" describes the act of release, not the depletion of the source. You can exude confidence all day without running out of it. Same principle the source (your haki pool) remains, you're just letting some out continuously.
Even if the drain is instant or not, it doesn't change the fact that Zoro only poured some of his haki. Not ALL of his haki. Enma draining Zoro's haki has no relation to Zoro able to coat his swords with CoC.
"after a couple of good attacks Zoro is done" You're just making this up. Nowhere it's stated in the manga Zoro only could do few attacks because Enma kept sucking his haki. You're just lying