You're right, but he addressed this near the bottom of the KS page: What Engine Does the Game Use? I have written the engine from scratch using C++ and OpenGL

You're right, but he addressed this near the bottom of the KS page: What Engine Does the Game Use? I have written the engine from scratch using C++ and OpenGL... Most importantly, the engine is designed completely around procedural paradigms, and this influences the architecture of the engine in many ways. Every file is written with "everything procedural" in mind. So, not so much re-inventing the wheel, but creating his own paradigm in a new engine. Brave to attempt something so novel, but this is how progress is made. If nothing else, he has a (hopefully) solid game engine that could form the basis for any number of future games. Gavan Woolery's Voxel Quest is another example of someone striking out on their own to invent a new engine. Look how awesome that turned out!