Your understanding is correct. A captive cable assembly is a cable assembly that is terminated on one end with a USB Type-C plug and has a vendor-specific connect means (hardwired or custom detachable) on the opposite end. If you do not want to get USB-IF certification (that means USB-IF logo on your product), then you can do whatever you want.
USB-IF has some document you should read:
CMG1 is a fixed-firmware device with only 9-ball CSP package. CCG2 is more flexible. But we usually will not recommend customer do customized modification on cable firmware and we do not release source code for cable application.