It's not just because of that. Everything is steeped in OpenGL API. All the "fundamentals" are about OpenGL.
Another link floated to the top of HN discussion just now, and every sibgle link there is better than these "fundamentals": https://news.ycombinator.com/item?id=49022038 E.g. "Computer Graphics from Scratch" https://gabrielgambetta.com/computer-graphics-from-scratch/ or Mathematics for Computer Graphics https://www.amazon.co.uk/Mathematics-Computer-Graphics-John-...