Game is written in Unity, where performance was never a particularly strong point.
It's using Unity as it's renderer. The underlying physics for example are based on another engine separate from the renderer so the game "feels" good regardless of renderer framerate cuz that's running at a different rate anyway.
Though saying written in engine x is weird either which way, cuz Unity is/was C# so more accurately you would have said written in C# though I doubt they actually coded this game is C#.....but maybe.
Unity you say? Is there any graphically impressive game based on Unity? I don't recall.
They've mentioned it already that they are using unity as a base for the renderer but it's tweaked far enough that it isn't just Unity.
They have their own internal name for the engine even if the credits will say Unity.
ON TOPIC
Absolutely stunning game.
I know moon get same hate for workplace environment and all that jazz, but no doubt their mastery at artstyles is somewhere beyond.
I was wondering why the games isometric view was still able to give a sense of depth so clearly, them bending the whole game world to ensure you still get horizons is genius....dunno how I never clocked it.
And I'm always a sucker for good dynamic fluid simulations.....If your game has those I immediately give you a pass for having high system requirements.
Honorable mention for the dynamic lighting too.....umph, this game gonna be good.