then it could do so, and do it without Apple’s assistance:
The U.S. has Gone F&*%ing Mad
James Allworth

We don’t know this for sure. They may have had such a technology in the past, and that technology may no longer work on current versions of iOS.
… or as you imply, they may just prefer to challenge it in court to set a precedent rather than unlock the phone themselves, and may be using this particular case to do so because of its terrorist nature.