Ceasars asking germa to go with them meaning they actually got away from katakuri, it make since germa isn't no soft targets
I see two routes :
1)Cesar AOE move overwhelmed FS ,he could have took away Katakuri oxygen so he passed out .
No matter how strong you are that it's a neg diff if distracted.
2 )Cesar poisoned Katakuri family members so he decided to help them instead of fight more.
Without Smoothie around the poison cant be removed from their bodies they must be treated or RIP.