That's patently incorrect. Executive dysfunction, a hallmark of ADHD, often results in something called "task paralysis" wherein you're unable to begin a task, even though it weighs on your mind.

The advice that a person could be exhibiting this symptom because of a different cause (such as depression) is true, but people with ADHD experience this issue at a higher frequency than their more neurologically typical peers, and if it's a problem being caused by ADHD, and not depression, it can lead to depression as a comorbidity.