Wow this chapter also confirms that coby not being affected by boas fruit was a haki feat by him (since strong haki negates all) since seraphim boa turned York into stone and she was not attracted to seraphim boa.
Either Koby dodged Hancock attacks or negated them. Which is a feat either way
There's a reason why Oda showed EVERY that marine and pirates in the area were turned into stone except him.