It seems like because there's so much custom code, rather than editor features, the author might as well use a rendering library instead of the engine.
When you want to build games for many platforms at once, dealing with platform-specific rendering, sound, and input code just becomes tiring. Having an engine at the base gives you these for free (or at a cost, with Unity).
no, a rendering engine just handles rendering, they listed multiple other things. don't conflate your definition to mean more than it actually literally does.
using game engine's builtins for the things you don't want to abstract yourself makes complete sense, keep the game to "game logic"
