Yep, that was part of it. Unfortunately one person has to be handed responsibility/liability. That's actually the idea behind why middle managers get paid more. They're responsible for everything done under their watch.
If you read the full article you'll find out that:
"The inspector general faulted three case agents — one a DEA employee and two assigned to an agency task force — and one supervisor who were responsible for Chong's safety. It said the supervisor exercised poor judgment and violated DEA policy by assigning two of the agents to process evidence from the cell after Chong was found."
So, in this case, among all the other faults (and there were several), the supervisor on hand also failed to keep watch over the people being supervised. The boss didn't know what the employees were doing. It wasn't just a case of similarly-leveled employees all failing at cross-checking their task.