Reportedly it’s made from a kit that Radio Shack sold for decades. It includes a battery backup (9-volt battery).
Why’d he even need to plug it in?
He’s been called brilliant and cool for creating a clock. Doesn’t sound like he had any innovation in this project. The skill of putting it together has been demonstrated. I doubt many kids who made these over the years ever thought to take them to school to show their English teachers in the middle of class.
I made a somewhat similar looking device many, many years ago. (It was a three bit "computer".) I told my SCIENCE teacher about it, before I brought it in, and showed it in the SCIENCE class.