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I imagine the electronics in this device do quick samples of the voltage, quick enough that the 60 Hz sine wave would appear flat for the instant of each sample. One sample would be with no load, and the second would be with 15 amps flowing. The difference of the two voltage samples would then be represented on the tester display.

This tester 15 amp sample current would be on infrequently and for very short duration, just long enough to overcome the "average circuit capacitance", that might reasonably be expected, before taking the voltage sample.

Also, the load pulse is short enough and infrequent enough to limit the heat that the tester electronics must dissipate to manageable levels.

So, it seems to me, with all of a house's circuits having simultaneous testing happening, the individual tester reading would be unreliable if the timing of the load pulse is not identical tester to tester. One tester would be providing a load while another tester is trying to sample the no load voltage. The Vdrop on the panel feeder and/or service conductors would be seen by all the testers.

If the tester electronics do trigger the load pulse at exactly the same instant, then the pulse Vdrop of multiple testers would be additive in the feeder/service conductors. This would be the worst case, IMO, resulting in erroneously large Vdrop readings. Let's say there are 40 branch circuits tested, 16 at the 20 amp level and 24 at the 15 amp level, evenly divided on either side of the panel. The service conductors out to the transformer would see load pulses of 340 amps each.

Al Hildenbrand