It's a hand-crafted engine by the game devs I'm pretty sure. Modding this game is kinda intense and the devs discourage it because they want to be able to make large changes to the code without having to worry about the work that went into someone's mod that may have become popular or whatever. It's probably written in one of the C languages, maybe someone who knows more will respond with actual info, basically I'm just here to say good luck figuring that all out lol