I know HH as an advocate of “skis together” style, and I’m not about to enter into a discussion on this, it has been well covered before. But could it be that this drill encourages two things, the first is to keep the feet together, that’s obvious. The second is to ensure both feet are doing the same thing at the same time. Clearly in certain conditions, as outlined in the accompanying text, it’s essential that your feet operate in harmony. That’s irrespective of whether your feet are wedged together or apart.
Remember these are only drills, and do not always reflect a way to ski. Recall other drills we’ve been made to do, running gates on one ski, skiing with our hands on our knees or leaning as far forward/back as possible. At the time I’ve often thought “why am I doing this”, but it generally all makes sense later.