It bothers me that no one acknowledged that this can still be logically true. Of course if most people's ADHD goes away in childhood then it doesn't really logically mean anything to ask "most people you know with ADHD" because by definition, you will only be asking the subset of people for whom that isn't true.
You can't really survey the people for whom ADHD went away in childhood by asking all the people you know with ADHD because the people who would confirm the psychologist's claim are not going to be included in that survey.