The author is handling sprite rendering entirely on the CPU, he's only using OpenGL to put a bitmap on the window. Little bit unnecessary.

GLFW+GL provides a portable way to do this though. Otherwise you'd need to mess with platform-specific window systems.

(something like minifb would be a more minimal option though: https://github.com/emoon/minifb)

SDL is more than enough for something simple like this. You don't need to mess with OpenGL.

Yes with SDL Surfaces.

Textures would be better. Surfaces are raw data, but textures are optimized for the individual hardware.

SDL2 is pretty popular these days

One of my goals is to reuse parts of the engine for future projects where hardware acceleration might be used.

Exactly!