Zoro is close to Luffy in offensive power, but Luffy is way more durable due to his Fruit, and also a lot more evasive and versatile.
Since Zoro uses slashes, rather than punches, he naturally does more damage than Luffy does. However, Luffy's strongest attack right now would definitely defeat Zoro's strongest attack.
You can see this by Zoro defeating a flaming dragon much smaller than Kaido: Oda is showing that Zoro is close to Luffy, but, that Luffy's Haki is much more powerful now.
Once they both iron out their CoC, their attack power will get more similar.