Judging from the installed game files it seems to based on SDL2 and not Unity3D.
> We do all of our development in C# using a custom low-level “game engine”. At some point we need to do a writeup about it, it’s kind of interesting tech.
But I think they have at least one Unity game... or so I thought.
Also found this quote[1] from an AmA on reddit:
> We've used C# since SpaceChem. I can't imagine using anything else, honestly. Infinifactory and TIS-100 use Unity, while everything else uses some kind of lightweight SDL-backed C# engine.
> SHENZHEN I/O and Opus Magnum are both using a very minimalist C# "engine" inspired by some of Casey Muratori's ideas on game engines and game programming. It uses DirectX or OpenGL for graphics, and SDL for everything else. It's basically just one giant Update() loop, and is the greatest game engine I've ever had the pleasure to work with. Despite this, I would not advise novice game programmers to do the same.
[1] - https://www.reddit.com/r/IAmA/comments/78wv2h/im_zach_barth_...