Well, in this case there's no jailbreaks involved, so no code modification or patching. Obviously if you have root access on the device you can do "anything"; the "non-JB-ness" of this hack is what was particularly intriguing.
(also, if the ipa contains all data required for the IAP to work, a jailbroken device stands no chance -- any server checks could simply be NOP'ed out)
You have to NOP each program, as the code to check each program against the server (which is written by the 3rd party developer, which then talks to apple server), which is significantly more effort than if the verification with the apple server occurred on the device.
(also, if the ipa contains all data required for the IAP to work, a jailbroken device stands no chance -- any server checks could simply be NOP'ed out)