It depends on whether they failed to stand still because they simply didn't want to follow orders or because they couldn't. Not really enough information to judge on in this article, but I believe that its because they couldn't based on what I know of kids. Your romantic notion of these individualistic children who'd rather be concentrating on something else just doesnt gel with my experience with kids. In general they're eager to please, especially in a one off situations.
So for example I rarely concentrated very hard in class, but enjoyed to for 12 hour days at chess tournaments. I knew plenty of similar people, some of whom were diagnosed with things like ADHD.
I'm not saying all children are good at concentration. I doubt most of the kids who stand still well are very good at concentration. It's just not what is being tested for and not what schools really care about.
I don't think its that controversial to suggest that a child who can sit still longer has better self-discipline. Self-discipline isn't exactly the same as ability to concentrate, but as a simple indicator for young children its interesting to study.