I would be disappointed if I didn't understand Zoro, sure.
"all he had to do was turn his Haki on"
He learned all new Haki, the highest grade, by bonding with his sword.
In addition, he had to learn how to fight against King's body in order to do damage.
"Magma dragon wasn't a great move"
It's not... assuming the opponent knows how to cut fire...
It was a phenomenally drawn fight, Zoro's longest in the series and the strongest he's beaten so far.
King vs Zoro was in total 42 pages and mostly are just action, no drama, no running, Zoro analysing King fighting style which is one of the most interesting one, he use all 3 mode, hybird,zoah,normal mode in such a natural way that is so effective (even better than kaido, and much much better than Queen)
Look next chapter we will have 2 more pages to King vs Zoro so a total of 44 pages which is exactly 3 hole chapters for this fight.
You guys should try read this fight like in this link totally a different experience, no break, no jump plot line only Zoro vs King.
Is really one of the best fight in One Piece (more interesting than Luffy vs Kata):