This is Oda protecting Zoro and his fans at his best.
Zoro is basically being just a usefull support but you can use the excuse he is not going all out.
Imagine Zoro doing his best, as Law and Kid, use Ashura and still being as effective as them (= useless compared to Luffy).
There would be an uproar.
I'm certain he will go all out only after he will be able to use Enma properly and wound Kaido.
That's the only way to protect him
Don't get me wrong, Zoro is doing perfectly fine in front of two emperors but it's far from being as impressive or more impressive than Law and Kid, let alone Luffy.
Luffy is galaxies far away.