Read it carefully! It doesn't say 'ground' it says 'AGND'. It isn't ground as in Earth, it's the analog ground of the ADC circuit but it must NEVER be connected to mains wiring Earth or to any other truly grounded point. The circuit has no mains isolation so it should only be used in situations where none of it can be touched and it has no direct external connections, for example in a self-contained electricity meter.