Why does Yoruichi say, "No one there could have survived a fight against Byakuya," in Bleach chapter 120?
It was likely a situational fact.


Ichigo had resolved the fight between him and Kenpachi, so he was physically weak.  He wouldn't have been capable of fighting Byakuya in that state.
Yoruichi hadn't been in battle in over a hundred years.  While she (somewhat) maintained her Flash Step, she couldn't have engaged him in a head-on battle.
It was very unlikely for Ukitake to intervene in the fighting, him being a fellow Officer, and while Ukitake would have been interested in protecting his subordinate, he wouldn't have immediately sided with the outsiders against the growing threat to Soul Society.