You're right, phrased this way it's misguided.
The point of this book is to emphathize how many conditions result in ADHD-like symptoms and how easily ADHD is diagnosed nowadays, often wrongly, resulting in heavy non-required treatments easily prescribed to people in need for something else.
For example, long term (drug) addiction presents itself as ADHD. To make confusion worse, plenty ADHD sufferers tend to abuse and get addicted to pleasurable activities (drugs, food, porn, etc.), worsening their condition. [1] It feels a bit like a chicken and egg situation but the point is: ADHD is a measurable difference in the pre frontal cortex that _needs_ to have been present since birth. There is no such thing as adult onset ADHD.
1: https://static1.squarespace.com/static/545e77cae4b0719cb5ad4...