Hm. It derives its clock by exciting caesium molecules using a pair of lasers, though. Some sort of clever feedback system which keeps the laser excitation frequencies tuned very tightly to just either side of the critical resonance frequency of the molecules. So it is a genuine atomic clock.
I'm not explaining it well. Hang on...
It says "The SA.45s CSAC employs coherent population trapping (CPT) to interrogate an atomic frequency. A laser illuminates atoms in a resonance cell with polarized radiation at two sidebands separated by the atomic resonance frequency. The atoms are excited to a non-scattering coherent superposition state from which further scattering is suppressed. The small size and low power of the CSAC is enabled by a novel electronic architecture, in which much of the functionality of conventional atomic clocks has been implemented in firmware rather than hardware.
The SA.45s electronic hardware consists of a low-power digital-signal processor, a high-resolution microwave synthesizer, and analog signal processing. The microwave output is derived from a tunable crystal oscillator and is applied to the laser within the physics package to generate the two sidebands necessary for CPT interrogation. A photodetector detects light transmitted from the laser after it passes through the cesium vapor resonance cell. Based on the measured response of the atoms, the microprocessor adjusts the frequency of the crystal oscillator."
And then there's a whole lot of other stuff about how stable it is and why that is. It's all bloody clever, mind you. Still not accurate enough to calibrate the timing circuit of a Type 40 TARDIS though. For that you need beryllium.