Note that the author directly acknowledges the existence of ADHD in the book. His concerns are directed toward the pharmaceutical industry's push to increase sales of stimulant medications. From the article:

Schwarz has no doubt that A.D.H.D. is a valid clinical entity that causes real
suffering and deserves real treatment, as he makes clear in the first two
sentences of the book: “Attention deficit hyperactivity is real. Don’t let
anyone tell you otherwise.” But he believes that those who are disabled by the
condition deserve a wider range of treatment options than an endless litany of
stimulants with chirpy names like Vyvanse and Concerta.

In my experience, I think most patients, parents, and clinicians would readily agree that ADHD is over-diagnosed and over-medicated. It's surprisingly common on internet forums and threads like these to find people insisting that ADHD is over-diagnosed in others, but that their own case of ADHD is genuine. Or parents who believe that stimulants are over-prescribed in general, but that their own child's case is an exception.

Clinicians feel this pain perhaps most directly. Few patients or parents arrive at the clinician's office seeking a diagnosis for their attentional symptoms without already having self-diagnosed themselves or their children as having ADHD using online tests, often from the pharmaceutical companies. Even fewer patients are open to alternative suggestions for addressing their problems, having already decided that they want a prescription for stimulants, and they want the exact stimulant formulation they saw online or heard about from their friend. It's an uphill battle for the clinician to argue against this, as denying patients the medication they've decided they need for themselves is a recipe for angry complaints that increasingly-busy clinicians don't really have time for.

The pharmaceutical industry knows full well that their target market tends to self-diagnose and decide what they want before ever stepping foot in to the doctor's office, and their marketing material is geared to maximize that effect. The ADHD information on the Vyvanse website, for example, reads more like a horoscope than a list of symptoms. From the Vyvanse website:

Kids may make careless mistakes in schoolwork, have difficulty remaining focused in class, and seem not to listen when spoken to directly.

Teens may avoid homework and may be easily distracted by many things, including unrelated thoughts.

Adults may forget to keep appointments, pay bills, or return calls and avoid completing forms or reviewing paperwork.

Unfortunately I don't know what the solution might look like, as the blame is shared by patients, parents, clinicians, insurers, and pharmaceutical companies alike. The fact that we allow companies to market diagnoses and medication directly to people represents a massive conflict of interest, as this incentivizes them to appeal to the widest audience as possible.

Better education of patients and parents would go a long way toward dispelling some of the myths around ADHD. It's entirely normal and expected for teens and children to not want to do their homework and to be distracted at times, just as it's unrealistic to expect adults to never miss an appointment, bill, or phone call. Kids naturally want to play video games or browse YouTube instead of doing their homework. At some point these issues reach the level of ADHD, but that level is much higher than most of the population would like to believe when it comes to themselves or their children.