He wasn't breaking the ropes to prove what to the Fishmen? Koby wasn't just running away. He was freeing slaves and drawing pirates away from the escapees.
Luffy wasn't breaking them off himself cause he wanted the fishmen to see that he didn't kidnap shirahoshi. That she is actually being protected by him. That he wasn't the bad guy.
Coby had no such reason. He would have had much more mobility without the iron ball. He could have protected the slaves better without the chain and ball. He could have gotten to Hibari faster without it and they probably wouldn't have even been frozen by Kuzan lol. But he just didn't break them off. He had to carry them with his hands and run...
And yet he couldn't dodge? No one's saying he had to beat the crap out of them. Just run. Instead he got captured. By people far weaker than Shiryu, even though he's almost certainly far stronger than Koby.
BECAUSE HE GOT TRICKED BY WOMEN.
Sanji is horni. His sense goes out the window when women are involved. Coby is not like that. He should have known that it was a pirate posing as a civilian. Cause even Grus figured it out. You know, Grus, someone who isn't hyped to have broken coo like Coby does?
That wasn't a CoO feat. At least, that's not how I read it. If anything, it might have been an experience feat. And yes. It was Shiryu. Random fodder didn't land a hit on him. They distracted him while Shiryu went in to attack
Coo can sense the will of a persons action etc. So it was almost certainly a coo related thing. But Coby was just that naive I guess...