Okay, so what. Zoro gets healed and tells Marco that he would take on King? Zoro doesn't want to fight King, he only wants to fight Kaido
As stated here, Marco's current role is unclear:
https://worstgen.alwaysdata.net/for...oilers-discussion.14451/page-292#post-1482585. He never aimed at defeating calamities. He only wanted to stall them the time Chopper creates the cure, an objective that already ended, so it's not even sure if King is the one he is actually fighting now, but assuming it's the case, he seems moderately tired at best(from his battle against King and Queen), with his blue flames still active, so at the very least, he should still be a difficult fight for King if he can even win
Hence why i don't think Marco would actually be defeated here, unless temporarily captured with underhanded tactics like seastone. As stated in the post, he still has a plotline with Luffy, which may lead him to want to help him in some way like he did in MF, following Ace's will, and the Weevil possibility can't be dismissed as well as well
As for Zoro, his objective towards Kaido was before the rooftop events.
Zoro trust Luffy in dealing with Kaido now.
His priority was to come to help in the live floor
With King being the main enemy of the live floor, it creates a build-up to their fight