I haven't read the article but, depending on how the research was done, the results wouldn't surprise me. The Pfitz quote is key. Maximize your one a day training, then two a days will hold some benefit. Running twice a day just to run twice a day, when you haven't fully maximized your gains from that first run, won't do you much good.
In my experience, two a days can be very beneficial. However, a lot of people seem to do them just to do them without thinking of how best to do them in order to maximize the training benefit. This is where the problem comes in. Often, people actually take away from the benefit of the primary run in order to get in that second run, which could even be detrimental.
This might be an interesting topic for an article of my own…