What I've seen & heard is that most juries and jury members take their role seriously, are diligent, want to do it right, and do so to the best of their ability.
And in a lot of more common cases -- especially the more basic ones like those from centuries ago when this system was just getting started -- they do fine. Their ability to judge who's lying, who's not, etc. is as good as anyone else's. Its all human nature.
But when that system got expanded to very intricate cases of highly technical laws and subject matter, the fundamental premise of the system was stretched past its breaking point. I've seen some of the closing instructions for cases like this. Even as a person with a top notch graduate degree, and directly relevant experience, and the luxury of seeing the instructions in print, and being able to review them at my leisure at the computer while looking up any more confusing terms, it can still be very difficult to follow.
The chance of a random juror, whose training and experience is in other areas, and has to hear many instructions primarily verbally, and at great length in one sitting, and without any modern technologies for making it better, actually fully understanding the material, is essentially zero. And that's no knock on the juror.
The article could well be right that jurors misunderstood. In fact most of the time, they almost certainly do. Its a problem with the system and not with the individual.