What do you think the detainees would have said?
You can't possibly be as naive as you come off. Detainees make a career of lying to reporters and others.
From your article:
Investigators conducted 20 interviews with "suspects and witnesses," the Southern Command said.
The full paragraph in the original press release: (boldface mine)
During the course of the investigation, investigators conducted 20 separate interviews with suspects and witnesses, including detainee medical providers, and searched three JTF-Guantanamo records systems used to track detainee-guard interaction and allegations of possible abuse. The evidence did not support any of the allegations of mistreatment or harassment.