Your HO is wrong for the 03 and up. 02 and below your info about the owner programming a code is correct, but 03 and up they switched to reading the VIN from the BCM.
Nope, Not a dealer myth. Fact. The 2002 and up use a signal from the class 2 data buss to read the VIN coding inside the radio. If that number matches the vehicle VIN then the radio works, If not it locks up and doesn't work. No way for the owner to alter any of that, you need a dealer scan tool and the correct coding to alter the programming. The partial VIN is stored in the radios memory and is read by the vehicles computer. If that radio loses power or blows a fuse it is no big deal, when power is restored the radio will be scanned and work as it always did.
On the vehicles that had owner programmed codes, the owner could push a couple buttons and enter the code they got and give them to a dealer who would run them and then give them an unlock code. If this type radio loses power and locks up the above will work to unlock it.
Sorry but there is NO code to unlock that radio. It has to be reprogrammed to read some digits in the VIN number of the vehicle in order to operate. Only a dealer can do it and your going to need proof that it isn't stolen before they will do anything.