Why the fuck wouldn't oda show off Coby if he was that strong? Why would he need Garp to save him if he was that strong?
Occam's razor. The simplest explanation is Coby just ain't that strong.
Robin was captured. Arc princess.
Sanji. Same deal.
And all the other arc princess were captured figuratively at the very least by their problems.
Again, Coby will not fight Kuzan. Cause if he was able to, he would just do so now. No need to wait for Garp to die. And he sure as hell won't go from below commander level to Admiral level in like a month.
None of the BBp commanders were giving two shits about Coby escaping. They were worried about wrecking the buildings than fighting Coby lol. So he's below even the lowest of the BB commanders like Vasco of Wolf.
Coby could have escaped after perona released him. There was no indication of him being able to escape from the cells. The prisoners say that he could run away after getting freed. Not that he could break out on his own. If he could, he would have done so.