They also said that the participant won half the trials. How could the experimenters fool the subjects about the game if they didn't get any harsh sounds back?
The actual study seems to be behind a paywall. The article claims that "participants were told they won about half the trials", which I assume would be by blasting them with noise ("fingernails on a chalk board, dentist drills, and sirens"). Why would they need to explain which noise was used if they never used it (the player had no opponent)?