zoro doesn't know about yamato, he only knows that luffy got defeated, so he's definitely going back to the rooftop.
yamato is fighting kaido with the intent of just holding him back until luffy returns.
buying time only means that yamato won't be doing any damage to kaido.
the fight will need zoro stepping in to install the mentality of "fighting to win".
zoro has expressed on multiple occasions that he won't wait for luffy, and that he will fight to win: