Participating Android phones are hooked into two different alert systems. One is ShakeAlert, but that wasn't the one that offered advance warning here. The other is that phones that have opted into the Android Earthquake Alerts System use mass collective data from their accelerometers to form a rudimentary real-time seismology dataset. The scale of the network is used to filter the seismology readings from the everyday movement noise. It's the data from that which was able to trigger the alert so rapidly and beat the shockwave, which is why this is discussing Android phones in particular.