For instance, a game engine written entirely in LuaJIT, but allocating almost all data structures using CFFI, a very unusual, custom object-orientation DSL I designed myself, and that uses SDL3's SDL_gpu library to do all rendering, a ton of GLSL 4.6 compute shaders cross compiled to SPIR-V and Metal (which constraints what GLSL can be written), and is a hybrid tile-oriented and 3D engine.
Another example is my Emacs-like live-modifiable image-based editor, again in LuaJIT+CFFI, with a kqueue+atomic lock-free ring buffer+pthread based actor model, each running a separate LuaJIT interpreter, with a custom advicing system, event bus, async/await implementation riding on the event bus, a custom process management and IO library, and testing via Tmux.
Even DeepSeek V4 Flash mostly does fine with this, once it's built a few memories in the memory system. Let alone something like DSv4-Pro or GLM 5.2. All I can say is skill issue.