I interpret as the OP does. The court document asks Apple to lock the particular image to a particular serial number, so if all goes according to plan, the same image could not unlock other iPhones. Obviously, writing security code on a short deadline does not make for the best security, so that's one worry.

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". Before you know it, Apple will have to have an entire department to make these one-off images. Someone will say, "you know, you could save yourself a lot of time if you just made it work on any phone." Then that image will be leaked, and their security guarantees will be dead. (One might also worry about the DRM implications.)