It's commonly known that Pathan is an Iranian ethnic group between Afghanistan and Pakistan. They also called Pashtuns, the ones who speak Pashto.
Khocha, too, is used for Pashtuns. This is not clear if it's a specific group among the Pashtuns but that seems like a possibility. However, khocha is a relatively less used term and is a little debated as well, as some people say it's not a Pashto word, but coined by Punjabi speaking people.
Overall, though, we can safely say that Khocha Pathan is an Afghan, basically a Pashto native, and hence, the term Khochey Pathan ki Zubaan would mean 'Word of a Pashto/ Pathan', which is considered something irrevocable, something which he'd not go against, whatever happens.