In the limitations it says what Russell Barkley always points out[1] as the missing key component in ADHD and autism studies: "Fourth, because of our observational study design, we were unable to exclude the potential residual confounders because of unmeasured genetic and environmental factors."

Both ADHD and autism have genetic causes in 80% of cases and run in families, not measuring genetic factors makes this whole study a waste of time.

[1] https://www.youtube.com/watch?v=RJGn4j6QTiw