More that they use GPS to synchronize the clocks. Having your own atomic clock doesn’t really improve your accuracy except for within the single data center you

More that they use GPS to synchronize the clocks. Having your own atomic clock doesn’t really improve your accuracy except for within the single data center you have it deployed (although I’m sure there’s techniques for synchronizing with low bounds against nearby atomic clocks + GPS to get really tight bound so they don’t need one in every data center)