Saying Zoro's just coating his weapon is like saying Luffy's just coating his fist. Luffy has internal destruction Haki, but Zoro's done things with CoA that neither Luffy nor Hyogoro for that matter have ever shown the ability to replicate.
Zoro created a CoA barrier large enough to divert Hakai. He can coat his swords in so much CoA that they begin steaming. There is nothing suggesting Luffy would be capable of doing either of these two, despite having internal destruction Haki. Nothing also suggests Luffy's CoA has gotten any stronger since round 1 on the rooftop, and there we have direct comparisons between Luffy and Zoro's CoA.
Luffy with Internal Destruction Haki + G4 was hitting Kaido with attacks left right and centre, BM never said a thing. Zoro with CoA that wasn't even on max release yet attacked Kaido and BM was screaming to dodge. Common sense dictates Zoro's attack was stronger than anything Luffy had used up until that point (Flying Dragon Blaze). Again, Zoro's CoA was not even on max release yet and bro was only using a single sword.
Therefore it's pretty obvious that Luffy isn't touching Zoro's CoA on max release. Zoro's CoA is outright stronger than Luffy's and has been for some time. Zoro is able to vastly increase the density of the CoA he outputs whereas Luffy can't to the same extent. For the most part, Luffy only increases his rate of use (G4). The difference b/w KOH and G4 steam is that G4 CoA is the same strength CoA Luffy always uses, he's just using up a shit ton at once. Meanwhile Zoro's max release CoA significantly increases his attack stength.
Case in point: A normal Snakeman punch without the G4 ability is the same as any other hardened punch from Luffy outside of G4. Meanwhile, the same slash from Zoro that would cut a tree would now cut a cliff if he uses more CoA - we literally saw this when he first used Enma. The way their CoA works is completely different and Zoro's is stronger because of it.
