It happens more often than most can imagine, innocent people being worn down and confessing to crimes they didn't commit, just to stop the interrogations.
In this case, they were found not guilty of the rape. That does not mean they were not guilty of the attack, which was one of a series of violent incidents that occurred in the park that night involving a group of youths. At least one confessed to the attack, and admitted to groping the victim, but not raping her.