[Retr0spectrum]

I haven't read the full paper, so this is only speculation.

I expect they would have counted the number of times the volunteers claimed to have seen a photon, both when a photon was emitted, and when they only said that one was emitted.

If the detection rate is significantly higher for "real" emissions than for the "fake" emissions, then you can draw a statistically significant conclusion (aside from the fact that there were only 3 test subjects).