In no particular order, here are some factors that I think make Dr. Schubert's exercise successful:
1. No wrong answers. Dr. Schubert makes a big point of this in one of the videos.
2. It's playful. They're laughing and having fun.
3. They're singing rather than laboriously picking out the notes on the piano. That can come later. Singing is instant.
4. Dr. Schubert is liberal with compliments and encouragement.
5. Rather than trying to manage full chords as accompaniment, they use only one note. This student is clearly already thinking of the harmony and playing a chord root, but with a younger piano student, the accompaniment could be delayed until the melody was worked out.
6. They work on one short phrase at a time.
What other aspects do you see that make this work so well? How could you adapt it to piano students at a variety of levels?