There’s another reason that might explain the improved problem solving capabilities of the ‘mixed’ team: idea generation, i.e., creativity, is consistent with non-linear thinking. Left to our own devices, however, the natural tendency for people is to default to sequential and analytic mental processing, becuase that’s how we’re taught to solve problems in school. Analytic thinking is best applied to late stage problem solving, when we need to validate that our idea can work; it’s less conducive to early stage creativity, where free thinking, and an anything-goes-attitude, leads to improved ideation.
It seems to me that the distraction afforded by the ADHD kids helped the team as a whole from getting too bogged down in focused, analytic thinking; ergo, better problem solving. Plenty of studies out there that appear to support this contention, such as this one.