Zorro showing coc potential ever since he was a kid
Sanji stood up to dangerous pirates to not give up on his dream and survived starvation for months. CoC potential.
Nami held up for years an abusive situation with Arlong just so she could buy back her village. CoC potential.
Robin survived for 20 years alone just so she could find the poneglyphs. CoC Potential.
Franky freaking stood up to stop a fucking TRAIN and, half-dead, built himself a new body with scraps, came back to form his own gang. CoC Potential.
There are so many characters in One Piece with "resilience", that it gets kinda funny when you try to nitpick what is and what isn't "coc worthy" to suit your agenda.
WHat I'm talking about is just that CoC isn't about "training". Rayleigh himself said you don't "train CoC" like you do CoA and CoO.
Zoro didn't "train to unlock CoC".
He unlocked it in a shounen's classic "PU" moment in a fight.
So there isn't a reason why Koby can't do the same.
Haki blooms in hard times, wasn't it?