You said:

"But Apple's letter uses the expression "technique", which I think means they're worried the government will get another court to make them change the serial number and sign a new image "next time""

Apple's letter directly claims that the particular piece of software created to comply with this request will reduce the security of it's users. Obviously this means that Apple does not think that the SIF being hardcoded with the unique identifier of the phone (sufficiently) mitigates the risk.

"make no mistake: Building a version of iOS that bypasses security in this way would undeniably create a backdoor. And while the government may argue that its use would be limited to this case, there is no way to guarantee such control."

Having re-read the OP more carefully, I think ghshephard is making a different claim than you. He is pointing out Apple's arugment about the 'unprecedented use of the All Writs Act of 1789'. If Apple can be forced to compromise their security via court order like this, the FBI gains the power to force Apple and any other US company to insert backdoors / decrease security.

"If the government can use the All Writs Act to make it easier to unlock your iPhone, it would have the power to reach into anyone’s device to capture their data. The government could extend this breach of privacy and demand that Apple build surveillance software to intercept your messages, access your health records or financial data, track your location, or even access your phone’s microphone or camera without your knowledge."

> would only load and execute on the SUBJECT DEVICE

You're wrong because any image that can be installed on the SUBJECT DEVICE can be modified to be installed on OTHER DEVICES.