So your reason boils down to "Zoro isn't strong enough."
Ignoring for the moment we don't know exactly how strong Zoro really is, or how strong he will be by the time for the battle against Kaido, let's make something clear, no one, I repeat, no one besides Linlin is strong enough to directly take on Kaido in Wano. Oda's made it abundantly obvious how much more powerful Kaido is compared to Luffy, the strongest member of the alliance. Luffy will likely not defeat him alone. So what should Oda do? Come up with methods to help in his(Kaido's) defeat.
The first was to improve Luffy's Armament proficiency. Second was to include likely candidates for it, such as Kid. Another is Zoro.
Now why Zoro? Well, it just so happens Zoro is carrying the one sword to have left a scar on Kaido. Now the conditions which led to Kaido receiving it are unclear, but why mention the Kaido scarring bit if Zoro will not be involved with Kaido? The sword could've been hyped up in any number of ways. Yet Oda deliberately included that detail. Look up
Chekhov's gun because that's exactly what it is. Every element of a story you introduce must play a part in it. If Enma is said to be a sword to have hurt Kaido, then that factoid must come to play at some point in the main conflict. Considering how it was used to end a chapter, no less cap off a volume, it's a very important piece of info.
There is obvious allusions being made to Zoro's role in Kaido's defeat. How Oda plays with the panels and speech bubbles are clearly meant to send a message.
If you can't see the obvious similarities between these panels, then we might as well be done here.
Circling back to the "Zoro isn't strong enough" argument, how is he not strong enough? Because One Piece doesn't follow a linear power progression. Characters are defined by their traits and abilities. How things interact with eachother is somethimes more important than who can throw the strongest punch. Take Nami for example, who proved more effective against Cracker(A top commander of a Yonko) and his biscuit soldiers than Luffy did prior. Law was able to damage Doffy's innards through Gamma knife while Luffy frustratingly tried to keep up with him earlier. By connecting Zoro with Kaido, Oda is already sowing seeds in how he can damage him. That begins with Enma, a sword described as "cutting more than intended." What more can Oda do with this sword? Do we just leave it to sucking away a user's haki? Or is there more to it? Can the sword tap into the hidden haki reservoirs the user couldn't before? Does it channel and store the haki passed on to it? We don't know, but Oda's provided himself the avenue by constructing Enma the way he did.
There's a lot of which we don't know about it, and to go along with that, Zoro himself will not remain stagnant power wise, as he is set to have an apparent combat presence for the rest of the arc.