It likely had a highly directional ("high gain") antenna. It could well have been focused enough to trigger a flash cube or flash bulb even with very modest power on the other side of the attenuator.
As for the difference between an attenuator and a dummy load, a 70 dB attenuator with a cap on the output would make for a quite good dummy load. So it was likely that one device was used for both purposes.
I wasn't able to find a reference for the field strength necessary to trigger a flash bulb. Presumably some sailors had discovered the effect accidentally, and passed down the knowledge for just this type of opportunity.