It's not whether Coby will beat Hancock or not, it's about advancing the plot. Obviously Hancock will destroys him in a fight but Coby will talk things out since that's his specialty, and once Hancock learned about Coby's relation with Luffy, she will stop attacking him. Now the question is, if Fuji and SSG is indeed the commanders of the fleet that is going after Kuja Pirates, how will Hancock and Coby handle the situation? My guess is that after the talk, Coby will go with Hancock for a while to a different location or make a false report that he captures Hancock so Fuji and the others retreated back to HQ.
