Updated game engine question

I read that Slormancer was built in Gamemaker, but saw a comment that the devs recently moved to an updated game engine.

Does anyone know what that new engine is? Is it just a newer version of Gamemaker, or was it rebuilt on something like Unity?

Just bought this game about a week ago and I am blown away by how polished it is for a small indie team. And the pixel art is phenomenal.