I looked at the datasheet and under normal operation the overall impedance of the circuit would be too high to cause a continuity tester to beep. However, A forward bias resistance of less than a K Ohm wouldn't surprise me. Also, there is no indication of the internal circuit other than the output on the datasheet. There may well be other diodes or solid state components internally that would fool you under reverse or forward biasing.
Have you actually connected this device to a power supply and tested it under operating conditions?