Interesting point of view from one who's been there. Intuitively the comment makes a lot of sense.
In a way human multitasking resembles how single-threaded computer programs work. That is, we focus on one task at a time but tasks can be interleaved to give an illusion of doing several tasks at the same time. Managing several tasks requires remembering the next step to do when leaving one task to attend to another.
That suggests ADHD people lose track of where they left off when performing several tasks. Distraction plays a role, but I think problematic working memory is the major issue. Among ADHD researchers there's a saying, "people with ADHD know what to do but don't do what they know." ADHD task performance may improve with proper treatment.