Think of (fragment) shaders as programs that run in the GPU per-texture-pixel. There is some initial CPU/memory-bound work setting everything up (e.g. uploading textures to GPU RAM, compiling the program, sending vertex buffers...) and then per-frame you just call the shader with some parameters (e.g. character's current position) and the GPU does the rest.

In-CPU you mostly do the non-parallelizable work such as collisions, game logic, keyboard input, etc. which can be very light for a simple 2D platformer.

I used to do a lot of amateur gamedev with Python (pygame and pyglet). If most of the work happens on the GPU or external libraries, it is usually OK if you aren't doing anything stupid.

If I recall he also makes code available via Patreon.