not in the industry anymore but I'd argue that, as far as 3D engines go, truly understanding an engine implies a breadth of knowledge much wider that what was necessary to make "simple" (by the time it was tough) 3D textured triangles. Things got much more sophisticated... Now you need at least some algebra, you need to understand datastructures, light behaviour, memory management, etc. Sure you can learn on the spot, but it's going to take time.