Earlier quoted context omitted.
It isn't. the FBI asked for a specialised OS image that would have a check so that it could only work on this specific phone. Combined with Apple's encryption/signing of the update, this image could not be applied to any other device. So the "work" asked of Apple would produce something that would only work on the phone with the warrant
So Tim Cook was wrong? Look at what he wrote: "Specifically, the FBI wants us to make a new version of the iPhone operating system, circumventing several important security features, and install it on an iPhone recovered during the investigation. In the wrong hands, this software — which does not exist today — would have the potential to unlock any iPhone in someone’s physical possession. The FBI may use different wo…
The software asked for by the FBI would only work on one phone because it would both have a check for the specific phone, and be signed by Apple (so can't be modified).
The software would need to be modified and resigned by Apple to be used on another phone.