"But otherwise, her attention span was remarkable".

Here is my understanding (though I suspet there arew many variations):

If an ADD child is intersted in a topic or activity (and, I am saying this child was) they have a huge ability to focus. But, unfortunately most school systems require children to focus on many topics and activities, some of which they have little interest in focusing. There lies a problem for many children with ADD, as focusing on these becomes a challenge. In school systems sympathetic and structured to ADD, a child most often performs much better. But, I suspect few are so.

Children with an ADD condition should not be confused as being less intelligent than other children. In many cased their intelligence level is actually higher.