The engine used to build this game was a once-off creation, designed explicitly to run this particular title. This is not a _real_ game engine, this is a generic category.
Making your own engine is hard work. But it can be worth it if it's done right. I know a lot of people don't do this anymore, but in my opinion, I think more people should do it, even though they don't release anything with it. Just so that they know how everything works behind the curtains.
If I were playing a game, I would definitely prefer a custom made engine. It gives a game this flare that the creator has taken a long time to make the game you're playing, and helps to allow someone to appreciate the game as a work of art. Even though it takes a while, I even want to build my games on engines I have made.