Well, it's just bad journalism, cause what they are referring to is a quantum inertial navigation system, not a clock – it's just thousands of time more precise. The plan happen to also have an atomic clock (which is used to properly integrate inertia through time), without having to rely on external GPS (which is a clock).
You just have to get closer to the source and find better information: https://www.gpsworld.com/uk-government-tests-quantum-inertia...