If zoro didn't have to protect luffy's weak ass throughout the fight, and if he didn't immediately collapse after asura from damage that can wipe all 5 sn out, kaido would actually take him into consideration as a worthy opponent too.
But as we saw, zoro only attacked to protect luffy, and even his retaliation attack "flying dragon blaze" missed kaido just to save luffy's dignity.
Zoro went to the rooftop to fight kaido but he changed his goal to save his weakling captains ass.