Well, there are certainly kinds of code LLMs would struggle with, but people generally underestimate what LLMs are capable of.
E.g. Victor Taelin is implementing ultra-advanced programming language/runtime writing almost all code using LLM now. Runtime (HVM) is based on Interaction Calculus model which was only an obscure academic curiosity until Taelin started working on it. So a hypothesis that LLMs are only capable of copying bits of code from Stack Overflow shall be dismissed.
E.g. Victor Taelin is implementing ultra-advanced programming language/runtime writing almost all code using LLM now. Runtime (HVM) is based on Interaction Calculus model which was only an obscure academic curiosity until Taelin started working on it. So a hypothesis that LLMs are only capable of copying bits of code from Stack Overflow shall be dismissed.