Unity is written in C. That the game logic is written in C# is hardly relevant as you explain. In fact this guy might still embed C# at some point if he wishes

Unity is written in C. That the game logic is written in C# is hardly relevant as you explain. In fact this guy might still embed C# at some point if he wishes for his logic to be less painful to write. The author states at the beginning that his goal is writing an engine and only after that write a game. You are right on all other points though, most people working on game engines simply never get to the making a game part. There's simply too much to do. I spent a couple years' working together with a friend every now and then on a game+game engine in C#. We did get past the game engine phase because it was kept simple (ECS + box2d and simple monogame 3d renderer) but we eventually gave up because even though game logic was progressing swiftly assets were just a pain in the ass. Most low level engineers (as we are/were) underestimate the work it requires to get a proper asset pipeline set up. You're never going to build a 3d game if you can't load cheap 3rd party assets with their animations and materials. And then the level designer which is basically just another game in your game. We went with UE4 as a change of pace and man is it librating not to have to worry about the limitations of your homebrew engine anymore. In UE4 literally anything is possible (you have the full source after all) and its always less work.