It looks like they relied on pre-existing information in the healthcare system when recruiting, which of course doesn't rule out misdiagnosed BPD(a disorder that is thought to be over-diagnosed in women). But it does give some rigor to their selection. Furthermore they seem to have done thorough assessment for ADHD:

>Following recruitment, all participants were screened for ADHD regardless of if they had already had a pre-established diagnosis. To establish a baseline diagnosis of ADHD, we used the parent-administered Diagnostic Interview Schedule for Children, 4th ed. (DISC-IV) [55] and SNAP rating scale [52], Hinshaw, [54] for the diagnostic algorithm). Comparison girls could not meet diagnostic criteria for ADHD on either measure. Some comparison girls met criteria for internalizing disorders (3.4%) or disruptive behavior disorders (6.8%) at baseline, yet our goal was not to match ADHD participants on comorbid conditions but instead to obtain a representative comparison group. Exclusion criteria included intellectual disability, pervasive developmental disorders, psychosis, overt neurological disorder, lack of English spoken at home, and medical problems preventing summer camp participation. The final sample included 228 girls with ADHD-Combined presentation (n = 93) and ADHD-Inattentive presentation (n = 47), plus an age- and ethnicity-matched comparison sample (n = 88). Participants were ethnically diverse (53% White, 27% African American, 11% Latina, 9% Asian American), reflecting the composition of the San Francisco Bay Area in the 1990’s. Family income was slightly higher than the median local household income in the mid-1990s, yet income and educational attainment of families were highly variable, ranging from professional families to those receiving public assistance. On average, parents reported being married and living together (65.8%) at the baseline assessment.

That said, they admit that the choice focus on psychometrics to utilize the entirity of the instrument they used to measure them, instead of sperating the predictor domains of hyperactivity vs. impulsivity is potentially limiting. Though they encourage more research into that:

> Fifth, we did not separate predictor symptom domains of hyperactivity vs. impulsivity, as psychometrics are superior when using the full 9-item Hyperactivity/Impulsivity scale. As well, this measure is more consistent with the DSM’s layout of symptoms. Although we support the separation of theses symptoms in future research—see the excellent national analysis by Tiger et al. [27]—we elected to include the full 9-item scale.