Bad guess. The PRC holds a lot of dollars as their reserve currency; devaluing the dollar would just make themselves poorer.
North Korea makes sense. They have the means, motive and opportunity. While they're a financial basket case, they do have technical skill, and a whole f'n country with no one looking. They have al the technology they can beg or borrow from Russia and China, steal from the South, or buy from AQ Khan.
Actually it doesn't. The article points out that the mint has made almost 20 minute changes to the plates that create the $100 since it was made, and these changes show up in the counterfit bills. If it was North Korea why go through the trouble? Just keep cranking out the older bills.
On the other hand, if the U.S. intelligence agencies are trying to trace the flow of funding to terrorist groups, rogue nations, front organizations, what have you, then what better way to do it than to work with the Treasury to print a relatively small amount of currency with a barely perceptible flaw and then funnel these bills into one end of the channel and watch for where they turn up? Feed them into the pipeline in Syria, for example, and watch them come out in North Korea. Or feed them in in Saudi Arabia and watch them turn up in Afganistan. It also makes sense if you're going to use bills marked in different ways to track different cash flows then you're going to get a copy of the current plates and alter it for your purposes. That could explain how the Treasury department changes to the real $100s show up on the counterfit notes.
It really could be a brilliant plan of our intelligence agencies to track terrorist financing. In which case it could have just been completely compromised by McClatchy.