There's certainly coupled, obtuse, contorted code styles that the LLM will be unable to twister itself into (which is different from the coupled, obtuse code it generates itself). Don't pretend this is good code though, own that you're up to your neck in shit.

LLMs are pretty good at modifying well factored code. If you have a functional modular monolith, getting agents to add new functions and compose them into higher order functionality works pretty darn well.