In this example, the FBI is the "trusted third party", but by giving them access, we inevitably open access for everyone, as the system is no longer strongly secure. The trusted third party in the quote isn't asking for access for everybody either, but in the end that's what happens.
Apple isn't giving access. Apple would be required (by court, unless they manage to fight this off) to install a signed custom build of the OS in order to give access to that particular device. FBI would not have this build, nor a key to create their own signed custom build.
Yup, that's the FBI's pitch. The issue is that it sets a legal precedent as well as potentially leaking a backdoored iOS to the world. Yes I know "signed for a specific device", best of luck with that.
