That is quite easy to reconcile: "ADHD has no negative effect on intelligence, when accounting for the impaired working memory". It is something you do all the time in psychology, you observe effects in presence or absence of other effects. Psychologists are smart enough to ask questions like "Now that we know that ADHD impairs working memory, does it further impair other factors of intelligence?" The answer seems to be no - which is important, because it implies that improvements in the working memory transition well into improvements in intelligence.
I give you the benefit of doubt here, but I'd wager that most of the non-academic folk that Dr. Barkley communicates with is intuitively aware of what I wrote above and able to form the right mental picture.