I’m guessing the chicoms
The article makes it sound like Russia is the source - maybe not so surprising, considering the meddlesome KGB is still very much alive and kicking.
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.
Or the Russians, or both. What better way to launder fake $100 bills than to let the Norks and Chinese gangs pass them around.....